On weekends he usually has time to spend his son. [ ]
A.at
B.for
C.with
D.in
答案:C
以点电荷q所在点为球心,距点电荷q的距离为r处的电场强度E等于:().
A.A
B.B
C.C
D.D
碱性焊条抗气孔的能力比酸性焊条强。( )