A:to find
B:finding
C:found
D:having found
有两个共点力,F1=2N,F2=4N,F3=5N,它们合力的最大值和最小值分别是多少是 ( )
2015年12月2日是第 个全国交通安全日 ,今年的主题是 。( )
— Air quality in our city is becoming worse and worse.—Yes. These days' PM2.5 readings are frightening. We must do something to ______air pollution.