A:C∶N为4∶1
B:C∶N为3∶1
C:隔绝空气
D:加大氮源、碳源的比例
The restaurant is nice and the food is not bad. I still prefer eating at home.
一辆自行车原价450元,现在只花了九成的钱。现价比原价便宜了( )元。