_{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