Which of the following methods is normally used to control the temperature of air distributed from individual cooling coils in an air conditioning system using a chilled water circulation system?（）

A.A room thermostat activates an electric heater at each individual chilled air outlet when necessary
B.Varying the number of passes the air makes across the coils
C.Varying the temperature of the water through the coils
D.Varying the quantity of water flowing through the coils