A1. If there are three cash registers open at the ticket counter, visitors have to wait 15 minutes to buy tickets. By how many minutes is the waiting time reduced if two more cash registers are opened?
(A) 3
(B) 5
(C) 6
(D) 7
(E) 10
A1. If 3 cash registers are open, the waiting time is 15 minutes. If only 1 cash register is open, the waiting time is 3 times longer, which is 45 minutes. If 5 cash registers are open, the waiting time is $\frac{45}{5}=9$ minutes. If two more cash registers are opened, the waiting time will be reduced by $15-9=6$ minutes. The correct answer is $C$.