Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Even though Ambulocetus swam by moving its body up and down, it did not have a backbone. B. The backbone of Ambulocetus, which allowed it to swim, provides evidence of its missing fluke. C. Although Ambulocetus had no fluke, its backbone structure shows that it swam like modern whales. D. By moving the rear parts of their bodies up and down, modern whales swim in a different way from the way Ambulocetus swam.