prep math 概率题

zcooles

A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students there are 60 sibling pairs, each consisting 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibiling pair?

A 3/40,000   B. 1/3,600  C. 9/2,000 D. 1/60   E. 1/15  
答案是A， 但是不会做。

有牛人会做的帮忙解答一下哈~