求教一道看不明白的排列组合题目 GWD14-7

Q7:

A certain university will select 1 of 7 candidates eligible to fill a position in the mathematics department and 2 of 10 candidates eligible to fill 2 identical positions in the computer science department.  If none of the candidates is eligible for a position in both departments, how many different sets of 3 candidates are there to fill the 3 positions?

A.     42

B.     70

C.     140

D.     165

E.      315

                                                                             Answer E

不明白怎么算出来的。。。。