二氧化硫是污染大气的有害物质之一,下列哪项可作为监测大气污染程度的指示植物( )
A:葫芦藓
B:铁线蕨
C:水绵
D:松树
——Did you hear that water in Tai Lake smelled terrible?——Yes.It ___________ because of people and factories.
有关我国地理位置的叙述,正确的是( )
a2=2a( a≠0 )