下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。
The Greatest Mystery of Whales
The whale is a mammal(哺乳动物)一warm—blooded，air-breathing，giving birth to its young alive，sucking them—and，like all mammals，originated on land，There are many signs of this．________(51)front f
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