Suppose a City Has 20 Citizens

Multiple Choice

Suppose a city has 20 citizens.The first 10 citizens each derive marginal benefit from traffic lights according to the function MB = 10 - Q,and the remaining 10 citizens each derive marginal benefit from traffic lights according to the function MB = 20 - Q.If traffic lights cost $20 each to produce,what is the efficient quantity of traffic lights?

A) 14 traffic lights
B) 16 traffic lights
C) 12 traffic lights
D) 9 traffic lights