The sooner one _____ to the new working conditions, the better it will be _______ his working efficiency.

A．adapts, for

B．adopts, to

C．adjusts, to

D．admits, for

平面内有两个定点F₁，F₂和一动点M，设命题甲，||MF₁|-|MF₂||是定值，命题乙：点M的轨迹是双曲线，则命题甲是命题乙的（　　）

A．充分但不必要条件

B．必要不充分条件

C．充要条件

D．既不充分也不必要条件