The need for an operation，especially an immediate operation， almost always comes as a ______ to the patient and his family.

A．result

B．happiness

C．disappointment

D．shock

一个袋子中放有红球、绿球若干个，黄球5个，如果袋子中任意摸出黄球的概率为0.25，那么袋子中共有球的个数为[ ]

A.15

B.18

C.20

D.25