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1 .listen to a conversation between a student and her marine biology professor.
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2 .Sorry to interrupt. Professor Lang,
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3 .looks like you\'re cramming for a test or something.
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4 .Oh, I subscribe to three marine biology journals.
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5 .Advances in my field come fast and furious.
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6 .I have to stay one step ahead of my students.
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7 .So what\'s on your mind, Sarah,
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8 .I wanted you to know that
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9 .letting me take the makeup midterm exam in your office yesterday really meant a lot.
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10 .As I mentioned, both of my parents had to go overseas for conferences last week.
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11 .That almost never happens.
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12 .So I had to go back home on Thursday and Friday to look after my little brother.
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13 .Glad it worked out.
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14 .So what did you think of the midterm?
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15 .I felt pretty confident,
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16 .except for one question,
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17 .maybe because I missed your Thursday lecture.
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18 .Oh, I thought we\'d arrange for you to attend that class session remotely.
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19 .The audiovisual system in the lecture halls allows students to do this, as long as their professor gives them the access code.
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20 .I didn\'t forget to email it to you. Did I?
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21 .No, I got your email and was planning to attend class through my laptop,
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22 .but my brother\'s teacher called that morning.
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23 .He fell on the playground and landed on his elbow.
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24 .Oh, no
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25 .yeah, it wasn\'t serious. But by the time I got back from the doctor\'s office, class was over.
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26 .That\'s a shame. Sarah,
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27 .I haven\'t granted your exam yet, but I\'m curious,
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28 .which question Did you have trouble with?
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29 .I wasn\'t sure about the marine mammals you asked about, like the blue whale,
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30 .why natural selection led it to grow to such a massive size, much bigger than the largest dinosaurs were.
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31 .I wrote that whales grow big because, well, because they can water so liberating.
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32 .Weights just not an issue if you don\'t have to support yourself on legs like land mammals do.
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33 .Yes, that\'s what intuition tells us.
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34 .But, and we did cover this in the class you missed,
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35 .water actually has the opposite effect.
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36 .Water doesn\'t allow mammals to grow big.
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37 .Marine mammals have to be big in order to survive.
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38 .Why？ body heat.
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39 .If the mammals were very small, they\'d lose too much body heat too fast.
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40 .They couldn\'t ingest enough calories to keep warm.
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41 .Bigger mammal bodies retain more body heat,
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42 .but there\'s a limit on the upper end of the size spectrum too.
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43 .If the blue whale got any bigger, it wouldn\'t be able to consume enough calories to support its massive body.
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44 .Fish don\'t have this problem, by the way,
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45 .their metabolism differs significantly from that of mammals.
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