What was he doing ______ his father _______ back?

A. while; came          B. when; came              C. when; was coming

一个盒子里装有10个红球，3个白球，1个黑球，摸出（    ）球的可能性最大，摸出（    ）球的可能性次之，摸出（    ）球的可能性最小。