我国______部人口密度大，______部人口密度小，大致以黑龙江的________市到云南的________为界。
东；西；黑河；腾冲
Only after Mary read her composition the second time __________ the spelling mistake.
A．did she notice
B．she noticed
C．does she notice
D．she has noticed
发电机失磁的现象有哪些？
