Today David is busy doing his homework. So he has no time ____ (play) computer games.
一只不透明的布袋中有三种小球(除颜色以外没有任何区别),分别是2个红球,3个白球和5个黑球,搅匀之后,每次摸出一只小球不放回.在连续2次摸出的都是黑球的情况下,第3次摸出黑球的概率是 ____
探究“光对鼠妇生活的影响”时: