[单项选择]

When my interest shifted from space to the sea, I never expected it would cause such confusion among my friends, yet I can understand their feelings. As I have been writing and talking about space flight for the best part of 20 years, a sudden switch of interest to the depth of the sea doer seem peculiar. To explain, I’d like to share my reasons behind this unusual change of mind. The first excuse I give is an economic one. Underwater exploration is so much cheaper than space flight. The first round-trip ticket to the moon is going to cost at least 10 billion dollars if you include research and development. By the end of this century, the cost will be down to a few million. On the other hand, the diving suit and a set of basic tools needed for skin-diving can be bought for 20 dollars. My second argument is more philosophical. The ocean, surprisingly enough, has many things in common with space. In their different ways, both sea and space are equally hostile. If we wish to s
A. Explain the reason of the shift of interest.
B. Tell us something about the sea.
C. Tell us something about the space.
D. Encourage people to investigate the sea.

When my interest shifted from space to the sea, I never expected it would cause such confusion among my friends, yet I can understand their feelings. As I have been writing and talking about space flight for the best part of 20 years, a sudden switch of interest to the depth of the sea doer seem peculiar. To explain, I’d like to share my reasons behind this unusual change of mind. The first excuse I give is an economic one. Underwater exploration is so much cheaper than space flight. The first round-trip ticket to the moon is going to cost at least 10 billion dollars if you include research and development. By the end of this century, the cost will be down to a few million. On the other hand, the diving suit and a set of basic tools needed for skin-diving can be bought for 20 dollars. My second argument is more philosophical. The ocean, surprisingly enough, has many things in common with space. In their different ways, both sea and space are equally hostile. If we wish to s
A. Confused.
B. Frightened.
C. Nervous.
D. Shaky.

The story that traces life from sea to land then into the multiple niches that exist there for a great variety of living things is a fascinating one, but far too detailed for our purposes. One key point for us in that story is the emergence of the biological class of animals that are called mammals.
Mammals have a number of features that distinguish them from the reptiles from which they developed. They are warm-blooded; that is, they have a system of temperature control that keeps the body at a constant temperature. Mammals have a set of teeth of different shapes that serve different functions such as cutting, gouging, and grinding. Young mammals spend their earliest days of development shielded within the mother’s body and are then born alive, rather than hatching from eggs. In addition, after birth they are nourished by milk provided by the mother’s mammary glands. The enforced association between mother and infant provides an opportunity for learning that
A. they are hand-feeders
B. of their anatomical features
C. they are arboreal
D. they depend on each other