Suppose that the marginal benefit of having a security guard at the airport is MB_{1},at the bank is MB_{2},and at the mall is MB_{3}.The marginal cost of assigning security guard is the same for all three places,and it is represented by MC_{2}.
The optimal number of security guards at the airport is ____,the optimal number of security guards at the bank is _______,and the optimal number of security guards at the mall is ______.
A) 3;3;6
B) 6;4;3
C) 5;4;2
D) 6;5;4