过点A(4,a)与B(5,b)的直线与直线y=x+m平行,则|AB|= ( )
A:6
B:2
C:2
D:不确定
已知数列{an}满足a1=0,an+1=an-33an+1n∈N,则a20等于( )
取用48mL液体时,最好选用的仪器是( )