Since there is such an abundance of
food in the sea, it is understandable that some of the efficient, highly
adaptable, warm-blooded mammals that {{U}} (67) {{/U}} on land should
have returned to the sea. Those that {{U}} (68) {{/U}} have flourished.
Within about 50 million years--{{U}} (69) {{/U}} time at all,
geologically speaking--one of the four kinds of mammals that has {{U}} (70)
{{/U}} to a marine environment has developed into the largest of all animal
{{U}} (71) {{/U}}, the whale. A second kind, the seal, has produced what
is probably the greatest population of large carnivorous mammals on
Earth. This suggests that these "top dogs" of the ocean are {{U}} (72) {{/U}} and multiplying. {{U}} (73) {{/U}}, such has not been the case, at ieast not for the last 150 years. Trouble has closed in {{U}} (74) {{/U}} these mammals in t A. Therefore B. However C. Otherwise D. Thus 我来回答: 提交
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