分类相加、分步相乘；先想清"完成一件事"的定义。
We have two urns, I and II. Urn I contains 2 black balls and 3 white balls. Urn II contains 1 black ball and 1 white …
两个罐子，随手挑一个，摸出一个黑球。问：这个球更可能来自哪个罐子？
