为使接收机输出端的有用信号达到规定的接收质量，在规定的条件下所确定的接收机输入端的有用信号与无用信号的最小比值为（）。
参考答案：射频保护比
下列关于平抛运动的说法正确的是 [ ]
A、平抛运动是匀速运动
B、平抛运动是匀变速曲线运动
C、平抛运动是非匀变速运动
D、平抛运动在水平方向是匀加速直线运动
I ______ basketball with him yesterday afternoon. He did much better than me. [ ]
A. was playing
B. played
C. would play
D. had played
