问题
单项选择题
A jar contains only x black balls and y white balls. One ball is drawn randomly from the jar and is not replaced. A second ball is then drawn randomly from the jar. What is the probability that the first ball drawn is black and the second ball drawn is white ？（）
A.A
B.B
C.C
D.D
E.E
答案
参考答案：E
解析：
第一次为黑球的概率是[*]，第二次为白球的概率是[*]，因此第一次为黑球而第二次为白球的概率为：
[*]