某机要室电动门的控制电路有两把钥匙（其实就是控制电路的开关），分别由两名工作人员保管，单把钥匙无法打开。如图所示的电路，符合上述要求的是：[ ]

A.

B.

C.

D.

To solve the traffic problem, more and more bridges ____________ across the river every year.

A．were built

B．will build

C．are built

D．are building