13: Dolphins lack vocal cords, nevertheless creating sounds: a complicated system of whistles, Squeaks, moans, trills, and clicks are produced by sphincter muscles inside the blowhole. A: nevertheless creating sounds: a complicated system of whistles, squeaks, moans, trills, and clicks are produced by B: nevertheless creating sounds: they produce a complicated system of whistles, squeaks, moans, trills, and clicks with C: but they do create sounds, producing a complicated system of whistles, squeaks, moans, trills, and clicks being produced D: but they do create sounds, a complicated system of whistles, squeaks, moans, trills, and clicks being produced E: but nevertheless creating sounds: a complicated system of whistles, squeaks, moans, trills, and clicks produced by 这道题没有人问过。参考答案是E，可是but 和nevertheless在一起意思表达重复。 D和E是我最先排除的。正确答案应该是B吧 | 
