Great changes __________ in my hometown since 1990.
A:have been taken place
B:have taken place
C:take place
D:had taken place
已知a>0,x、y满足约束条件{x≥1x+y≤3y≥ax-3,若z=2x+y的最小值为32,则a= ( )
实验室用加热KMnO4固体的方法制取氧气,下列仪器中不需要的是( )