与其他动物相比，哺乳动物后代成活率较高的主要原因是（　　）

A．用肺呼吸

B．胎生哺乳

C．心脏四腔

D．体表被毛

用所给词的正确形式。

1. When I ________(give) you a present, you must open it at once.

2. American people eat lots of ________ (hamburger) everyday.

3. Can he ________ (help) me ________ (do) the work?

4. It's too difficult ________ (understand)what you are trying ________ (say).

5. He could see the smoke ________ (rise) from the chimney (烟囱) when he

    ________ (walk) past your house at this time yesterday.