Mr.Li often drives his car to work,but he rides a bike there.
A:usually
B:sometimes
C:always
D:often
下列语句中,命题的个数为( )
①空集是任何非空集合的真子集;②三角函数是周期函数吗?③若x∈R,则x2+4x+7>0;④指数函数的图象真漂亮!
下列表述中,所指的速度为平均速度的是( )