填空题

阅读下面这篇短文，短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为规定段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Every year thousands of grey whales make the longest migration of any animal, traveling 7,000 miles each way between the Arctic and Baja, California. The grey whales spend the long summer days in their Arctic feeding grounds in the Bering Sea between Alaska and Russia. Unlike a fish, the whale is warm-blooded and must therefore maintain a relatively high internal body temperature. In the cold Arctic waters the grey whale is protected by an outer layer of blubber which averages six inches in thickness. Other whales such as the Greenland Night whale have been found with a two-foot thick layer of blubber covering their bodies.
2. During these summer months, in the Arctic the grey whales fatten themselves by consuming enormous quantities of small shrimp-like animals called amphipods. Recent observations of a young grey whale, Gigi Ⅱ, held in captivity at Sea World in San Diego during 1971 and 1972, suggest that the grey whale feeds by sweeping its enormous head over the bottom. The amphipods on which it feeds are either stirred off the bottom or leap off the bottom to escape. These animals and the surrounding water are sucked into the whale’s mouth. As the water is expelled from the mouth, it passes through coarse baleen fibres. The small animals are trapped and swallowed.
3. In October as the days get shorter and ice begins to form over the Arctic feeding grounds, the California grey whale begins its long journey south to the warm waters of Baja, California. During this three-month long trip the whales travelling in groups stay close to the shore of North America, swimming both day and night and averaging abut 100 miles per day.
4 During the trip south the grey whales that are sexually mature, at least three years old, and not pregnant already, form mating groups. These groups are composed of three whales, two males and a female. The dominant male couples with the female while the second male is kept busy positioning the two whales on their sides facing each other and keeping them together during the sex act. This is no small job, since each whale can be fifty feet long and weigh forty tons.
5. During December and January the grey whales arrive at the warm lagoons along the coast of Baja, California. The whales swim miles inland along narrow shallow channels. These channels are the breeding grounds of the California grey whale. The calf has been gestating in the pregnant female for the last thirteen months, that is, since her last journey south. The expectant cow is aided in the birth of the calf by another female that acts as a midwife. At birth the calf sinks toward the sea floor. Being a mammal the whale must breathe at the surface. The midwife guides the baby whale to the surface for its first gulp of air. The calf then finds its mother’s nipples and rich whale milk is forced into its mouth. During the next two months the calf will grow twenty feet and double its weight.
6. In March the whales begin their long journey north to the Arctic. The newly-pregnant females leave first, following by the males and immature females. The last to leave the warm waters of Baja are the females and their calves. The whales arrive at their Arctic feeding ground in June. Many scientists believe that during this entire eight month long, 14,000 mile journey, the California grey whale does not feed at all! A layer of blubber approximately six inches thick ______.