Why have the Americans developed space shuttle according to the passage a. It is a first step to explore outer space. b. It is a follow-up action of the Apollo moon landings. c. It is a compromise to the exorbitant human expeditions to the Mars. d. It is a compensation for the wishful thinking of human landing on the Mars.

在潮湿的环境配电盘的二次线路绝缘电阻值可降低为（）兆欧。

A.1

B.0.22

C.0.5

D.2