物体通过凸透镜可以在光屏上形成倒立缩小的实像,下列哪个光学仪器应用了这种成原理 ( )
A:照相机
B:放大镜
C:幻灯机
D:汽车后视镜
—The man at the gate _______ be our headmaster. He is always standing there every morning.
—No, it _______ be him. He is holding a meeting in the office now.
近年我国每年约增加1200多万人,主要原因是