利用次级点火波形重叠可诊断出（）。
A.分电器凸轮磨损情况
B.断电器触电闭合角的大小
C.混合气浓度是否合适
D.喷油器是否堵塞
参考答案：A, B
My sister_______ the League last month and she has decided to _______ the activity to help the aged next week.
A．joined; join in
B．took part in; join
C．joined; attend
D．joined in; join in
电镜下，尼氏体是由平行排列的粗面内质网和游离核糖体构成，它可合成（）。
