下列有关空气各成分的说法错误的是( )
氧气能供给生物呼吸,支持燃料燃烧
二氧化碳是植物进行光合作用必需的物质
氮气的化学性质不活泼,可用作食品防腐
稀有气体通电发出各种不同颜色的光是化学变化
do you visit your grandmother in the countryside?
Once a month.( )
How far
How long
How soon
How often
Mr.Smith be in the office. Because he has gone to Chengdu on business.( )
mustn't
shouldn't
can't
needn't
The young man was of all the performers.( )
more creative
much creative
the most creative
the creativest
It's important for us a healthy lifestyle.( )
to keep
keep
keeping
kept
They worked for days, and finished the task successfully.( )
twelve; two
twelve; second
twelfth; two
twelfth; second