^_—There goes the bell .  

^_—It's time for class. Let's stop __________.[ ]

A. talk

B. to talk

C. talking

D. not talking

如图所示，为某一家庭的遗传系谱图，父母均为正常，其儿子为白化病患者，则该父母所生第二个小孩为白化病男孩机率为（）

A．0

B．1/4

C．1/8

D．1/2