黃金閱讀 The Origins of Cetaceans - 英檢

It should be obvious that cetaceans—whales, porpoises, and dolphins-are
mammals. They breathe through lungs, not through gills, and give birth to
live young. Their streamlined bodies, the absence of hind legs, and the
presence of a fluke(1) and blowhole(2) cannot disguise their affinities
with land dwelling mammals.

1. In paragraph 1, what does the author say about the presence of a
blowhole in cetaceans?

(1)It clearly indicates that cetaceans are mammals.
(2)It cannot conceal the fact that cetaceans are mammals.
(3)It is the main difference between cetaceans and land-dwelling mammals.
(4)It cannot yield clues about the origins of cetaceans.

答案是(2)

為什麼答案不是(3)呢? 原文中意思應該是即使cetaceans有blowhole，
但是cetaceans和land dwelling mammals還是有關係

cetaceans是mammals文章不是一開頭就已經講了嗎

謝謝大家!

--