甲氨蝶呤抗肿瘤的主要机制是（）。

A.抑制二氢叶酸合成酶

B.抑制二氢叶酸还原酶

C.破坏DNA结构和功能

D.嵌入DNA干扰转录RNA

E.干扰蛋白质合成

When launching a lifeboat in the rough sea, the davit gears should be released（）.

A.before the boat is lowered from the stowed position

B.as the boat fall blocks break clear of the davit head

C.before the boat is lowered from the embarkation level

D.after the boat is released into the water