Some of the students are afraid of the teachers          strict with them.

 You can only get out of something ______ you put into it.

（2014年北京市西城区高三5月二模试题）31. A person who has never tasted bitter won’t know ______ sweet is.

Although the teacher had told Bob the history of telephone, Bob didn’t know when ____. 

Lucy said she ______________to my city the next month.