视频内窥镜是利用（）作为光电转换元件的
参考答案：电荷耦合器
Once education is made free，many children who have dropped out of school .
A．will come back
B．came back
C．have come back
D．had come back
车轮的轮缘贴着轨道侧面运行，车轮的轮缘与轨道的侧面有轻微的摩擦这属啃轨现象。（）
