H₂、O₂和N₂的混合气体20g点燃，充分反应后生成18g水，则剩余气体不可能是（　　）

下面用数学语言叙述代数式$\frac{1}{a}$﹣b，其中表达正确的是（　　）

用单词的适当形式填空
(1)It's about twenty ____ （分钟）walk from here to the post office.
(2)The ____ (音乐人) like guitars and violins
(3)Lots of____ (村民) never leave the village.
(4)He ____ (想念) his family every day.
(5)He has an English test tomorrow , so he ____  (学习)  for the test now.