中度牙弓拥挤为()
A.2~4mm
B.3~5mm
C.5~8mm
D.8~10mm
E.以上都不是
参考答案:C
-- ________ will you come back from the library, Linda?
-- In an hour.
A.What time
B.How often
C.How soon
D.How long
同步水轮发电机的频率和转速有什么关系?其“同步”是什么含义?