B．Great efforts should be made to save energy.

　　C．Human activities should be environmentally friendly.

　　D．New equipment should be introduced for space study.

　　B篇【2017浙江卷】B

　　B

　　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 blood-filled 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 are 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 grown-ups?

　　Baby (or “milk") teeth do not last long; they fall out to make room for bigger, stronger adult teeth later oil 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 are 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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