Below is a selection from a popular scie

问题：

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 color. 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?

Which works harder 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 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.

Do old people shrink as they age?

Yes and no. Many people do get shorter as they age. But, when they do, it isn't because they're shrinking all over. They simply lose height as their spine(脊柱) becomes shorter and more curved due to disuse and the effects of gravity. Many (but not all) men and women do lose height as they get older. Men lose an average of 3-4 cm in height as they age, while women may lose 5 cm or more. If you live to be 200 years old, would you keep shrinking till you were, like 60 cm tall, like a little boy again? No, because old people don't really shrink! It is not that they are growing backwards — their legs, arms and backbones getting shorter. When they do get shorter, it's because the spine has shortened a little or, more often, become more bent and curved.

Why does spinning（旋转） make you dizzy (眩晕的)?

Because your brain gets confused between what you're seeing and what you're feeling. The brain senses that you’re spinning using special gravity-and-motion-sensing organs in your inner ear, which work together with your eyes to keep your vision and balance stable. But when you suddenly stop spinning the system goes out of control, and your brain thinks you're moving while you're not!

Where do feelings and emotions come from?

Mostly from an ancient part of the brain called the limbic system. All mammals have this brain area — from mice to dogs, cats, and humans. So all mammals feel basic emotions like fear, pain and pleasure. But since human feelings also involve other newer bits of the brain, we feel more complex emotions than any other animal on the planet.

If exercise wears you out, how can it be good for you?

Because our bodies adapt to everything we do to them. And as far as your body is concerned, it’s "use it, or lose it”! It's not that exercise makes you healthy; it's more that a lack of exercise leaves your body weak and easily affected by disease.

小题1:What is the color of blood in a vein near the surface of the skin?

A．Blue.

B．Light yellow.

C．Red.

D．Dark reddish purple.小题2:Why do some old people look a little shrunken as they age?

A．Because their spine is in active use.

B．Because they are more easily affected by gravity.

C．Because they keep growing backwards.

D．Because their spine becomes more bent.小题3:Which of the following statements about our brain is true?

A．In the long run, our brain probably works harder than our heart.

B．When our brain senses the spinning, we will fell dizzy.

C．The brains of the other mammals are as complex as those of humans.

D．Our feelings and emotions come from the most developed area in our brain.小题4:What is the main purpose of the selection?

A．To give advice on how to stay healthy.

B．To provide information about our body.

C．To challenge new findings in medical research.

D．To report the latest discoveries in medical science.