At first, being scared, the bird fluttered about the cage, but finally it ________ in its new house.[ ]

A. settled down    

B. settled up    

C. set  up      

D. set down

增压透平膨胀机启动前的准备工作是增压膨胀机的仪控、电控系统及油、水正常，增压机回流阀关闭。