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?
42
70
140
165
315
排列组合
select 1 of 7 candidates eligible to fill a position in the mathematics department C1/7
2 of 10 candidates eligible to fill 2 identical positions in the computer science department C2/10
C1/7*C2/10=315