空气是一种宝贵的自然资源,下列气体不可直接从空气中分离获得的是( )
A:用作医疗急救的氧气
B:用作焊接保护气的稀有气体
C:用作食品防腐的氮气
D:用作高能燃料的氢气
看图片,写单词。
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—— __________ do you chat by QQ with your friend in Shanghai every week?——Two hours. I have much homework to do every day.