电动机起动发动机是用电动机作动力，以（）为动源。
A、发电机
B、蓄电池
C、整流器
D、飞轮
参考答案：B
—Lucy has gone to China for her holiday.
—Really?When________back?[ ]
A. is she coming
B. did she come
C. had she come
D. has she come
核酸类酶
