2016高考英语二轮阅读理解八十集之连载训练（44）

　　阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。

　　Below is a selection from a popular science book.

　　If blood is red，why are veins(静脉) blue?

　　Actually，veins are not blue at all.They are more of a clear，yellowish colour.Although blood looks red when it's outside the body，when it's sitting in a vein near the surface of the skin，it's more of a dark reddish purple colour.At the right depth，these blood­filled veins reflect less red light than the surrounding skin，making them look blue by comparison.

　　Which works harder，your heart or your brain?

　　That kind of depends on whether you're busy thinking or busy exercising.Your heart works up to three times harder during exercise，and shifts enough blood over a lifetime to fill a supertanker.But，in the long run，your brain probably tips it，because even when you're sitting still your brain is using twice as much energy as your heart，and it takes four to five times as much blood to feed it.

　　Why do teeth fall out，and why don't they grow back in grown­ups?

　　Baby (or “milk”) teeth do not last long；they fall out to make room for bigger，stronger adult teeth later on.Adult teeth fall out when they become damaged，decayed and infected by bacteria.Once this second set of teeth has grown in，you're done.When they're gone，they're gone.This is because nature figures you're set for life，and what controls regrowth of your teeth switches off.

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