单纯性下肢静脉曲张的主要病因是()。

A．妊娠

B．动静脉瘘

C．深静脉受阻

D．静脉壁发育不良

E．腹内压增高

Careful testing is necessary before any more experiments can continue with the exploring submarine.

A. needful
B. in need of
C. necessity

D. of necessity