He's never been late for class, ______ he? [ ]
A. is
B. isn't
C. has
D. hasn't
答案:A
单一电阻元件的正弦交流电路中,复阻抗Z=()。
盎司和蒲式耳分别属于()单位。
A.重量;长度
B.重量;容积
C.长度;容积
D.容积;长度