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.

ZPW2000轨道电路在轨道电路任一处轨面用标准分路线进行分路时（电气绝缘节区域死区段除外，进行电压和电流测试。接收器的限入残压不应大于（），轨道继电器残压不应大于5V，并可靠落下。

A、100mV

B、120mV

C、140mV

D、160mV