题目

为了节约用电，很多楼梯上方的过道灯是由光控开关和声控开关同时控制的。光控开关受到光照时断开，声控开关接收到声音时闭合，请将图的原理图连接完整，要求满足；在夜晚天黑时，接收到声音，灯就发光；在白天无论接收到多响的声音，灯都不亮。 答案：Long ago people never knew that population ________ a big problem. [　　] A．will beB．had been C．had becomeD．would be