问题如下：
We expect that 15% of high school students will join in the work next year. Assuming we select 5 high school students randomly ,the probability that exactly two of them will join in the work next year is equal to:
选项：
A.0.138.
B.0.114.
C.0.185.
解释：
A is correct.
[5! / 2!(5 - 2)!](0.15) 2(0.85)5 - 2 = 10*(0.0225)*(0.6141) = 0.1382.
老师看到题目的时候无从下手，不知道在考什么，请问怎么样才知道他是在考阶乘和binomial呢？
