At the railway station，he waved goodbye to his friend until the train was ________.

A．out of sight

B．out of reach

C．out of order

D．out of place

低压空气阻尼式时间继电器又称为气囊式时间继电器，它是利用空气阻尼的原理配合微动开关来产生延时效果，JS7-3A型空气阻尼式时间继电器的气囊在（）的位置。

A.2

B.3

C.4

D.5