用作
二氟甲烷是性能优异的环保产品，它可以代替会破坏臭氧层的“氟利昂”用作致冷剂，试判断二氟甲烷的结构简式（ ）．
二氟甲烷是性能优异的环保产品，它可以代替会破坏臭氧层的“氟利昂”用作致冷剂，试判断二氟甲烷的结构简式（ ）．
The pot _____ for _____ hot water.
The pot \_\_\_\__ for \_\_\_\__ hot water.
A _____ language is one which is still spoken by people in the course of their everyday lives.
A \_\_\_\__ language is one which is still spoken by people in the course of their everyday lives.
I woke up with _____ bad headache, yet by _____ evening the pain had gone.
I woke up with \_\_\_\__ bad headache, yet by \_\_\_\__ evening the pain had gone.
I was trying to get into the _____ bus when I heard a voice from behind.
I was trying to get into the \_\_\_\__ bus when I heard a voice from behind.