① What is the main purpose of paragraph A in the passage?
② How does paragraph B relate to paragraph A
③ Which of the following best describes the relationship between paragraphs A and B
④ Paragraph B expands on paragraph A by
具备举例关系的段落，一般的模式是 A 段是一个中心观点，而 B 段是例子的展开去支持 A 段落中的观点。
【paragraph 3】Many adaptations to this general problem are based on a resting stage that awaits favorable conditions. We will consider two examples from the vertebrates. The first is the red kangaroo. This marsupial inhabits the deserts of central Australia where the onset of rains and the resulting sudden growth of vegetation are extremely unpredictable. Obviously, it is advantageous for a kangaroo female to produce young at a time when plant productivity is sufficient to support her offspring. For such a relatively large mammal, however, gestation (the period of development during pregnancy) is so long that if a female waited to mate and carry the young until after the rains came, the favorable period might be past. The kangaroo’s life history adaptation to this problem involves the use of embryonic diapause during gestation (development in the uterus).
【paragraph 4】After a 31-day gestation period, the female gives birth to a tiny helpless young typical of marsupials. The newborn crawls into the mother’s pouch and attaches to a teat where it continues to grow and develop. After 235 days it leaves the pouch but remains with the mother and obtains milk from her. Two days after giving birth, the female mates again. The fertilized egg enters a 204-day period of diapause during which it remains in the uterus but does not attach. It then implants, and 31 days later, birth of the second young occurs. Note that the first young leaves the pouch at just this time. Again, the female mates, fertilization occurs, and another diapause follows. The eventual result is that at any one time, the female has three young at various stages of development one in diapause, one in the pouch, and one outside the pouch. Among other benefits, this allows her to freeze the development of an embryo during times of drought and food shortage until the offspring in the pouch is able to leave.
7. What is the main purpose of paragraph 4 in the passage?
A. To give the details of an adaptation mentioned in paragraph 3
B. To describe an adaptation different from the one explained in paragraph 3
C. To introduce an adaptation that is described in detail in paragraph 5
D. To discuss an adaptation that is not as successful as the one discussed in paragraph 5
第三段中提到两种有脊椎动物通过一个休息阶段等待好的环境来处理某个问题，其中一种是红袋鼠。段落顺着往下一直到本段尾句读到 The kangaroo’s life history adaptation to thisproblem involves the use of embryonic diapause during gestation (development in the uterus).该句中提到了红袋鼠具体的处理问题的方式是在孕期让胚胎休眠。因此该段主要提及红袋鼠处理问题的方式，段落的中心实则为尾句。
选项 A 表述第四段为第三段提到的一种适应提供详细的细节，符合两段关系，其中的 adaptation 即红袋鼠处理问题的方式的同义替换，因此直接判断为答案 A 选项。