题目内容：

Scientists have identified the order of the gene of the giraffe for the first time to help explain how the tallest animals on earth developed their remarkably long necks.

Being a giraffe is not easy. To pump blood two meters up from the chest to the brain calls for a powerful heart and twice the blood pressure of other mammals. Giraffes also need special safety system to let them bend down for a drink and raise their heads again without feeling Dizzy.

The animals* unique body structure has long been a puzzle for biologists, including Charles Darwin. Now, by comparing the gene of the giraffe with its closest relative, the short-necked okapi, scientists have solved part of the puzzle by studying changes in a small number of genes responsible both for regulating body shape and circulation. This suggests that the development of a long neck and a powerful heart went hand in hand, driven by a relatively small number of genetic changes.

The interpretation of the genetic factors behind the giraffe's remarkable heart system could also be instructive for human health, since the animals appear to avoid the kind of organ damage often found in people with high blood pressure.

The more fundamental question of why giraffes developed their long necks remains open, however. The apparently self-evident idea that it was to reach ever-higher food supplies has been challenged in the past 20 years by a competing belief that it is actually due to competition among fighting males for females.

Unlike long-necked birds, which have additional vertebrae (椎骨)，giraffes have the same seven vertebrae found in all mammals, although theirs are greatly extended.

1.A strong heart is very important for a giraffe because it .

A. keeps the giraffe safe in different environments.

B. pumps blood to the brain high above its body.

C. helps the giraffe find a drink at lower places.

D. keeps the blood pressure at a normal level.

2.Scientists compare the gene of the giraffe and the short-necked okapi because the latter .

A. has a quite special body structure.    B. has gone through few genetic changes.

C. is a puzzle worth further study.    D. is in the same family as the giraffe.

3.Which possibly caused the giraffe to develop a long neck?

A. Its attempts to attract females.    B. Its favorite food at high places.

C. Its need to avoid organ damage.    D. Its greatly-extended vertebrae.

4.What’s the author’s purpose of writing this passage?

A. To describe giraffes body structure.    B. To explain genetic changes in animals.

C. To introduce a new finding of science.    D. To challenge a belief of twenty years.