The Figure Below Shows the Marginal Benefit of Hiring a Security

The figure below shows the marginal benefit of hiring a security guard at the airport (MB_A) , the bank (MB_B) and City Hall (MB_C) _. The marginal cost of hiring a security guard is the same at all three places and is given by MC. The optimal number of security guards is ______ at the airport, ______ at the bank and ______ at City Hall.

A) 3; 2; 1
B) 2; 4; 6
C) 7; 6; 5
D) 6; 4; 2