Although police could be seen everywhere, they seemed to be searching the houses almost ________.

A．at ease

B．at random

C．at risk

D．at length

如图所示，在干燥的圆底烧瓶中充满某气体a，胶头滴管中吸有少量液体b，当把溶液b挤进烧瓶后，打开止水夹不能形成“喷泉”的组合是（　　）

A．A

B．B

C．C

D．D