A:____.
B:Good evening.
A:____?
B:I have dinner at half past six.
A:____ ?
B:No, I don't eat rice.____?
A:I have dinner at seven and I eat rice.
B:At home?
A:Yes.____?
B:I have dinner at home too.
今年春季以来,我国许多城市雾霾天气频发,PM2.5是造成雾霾天气的“元凶”之一.PM2.5是用来监测空气中下列哪项含量的( )