．—What’s the matter with the man hanging his head there? —Well, if you _______ know, he was caught stealing my bike.

 I wrote a letter to John for his help during my high school years，without which I ________  the college entrance exam.

 —Happy birthday!—Thank you！It's the best present I ________ for.(2012·江苏)

 Now 70 years old, the British physicist, Pendry can’t remember a time when he was not asking a question as a kid, joking that he ________ his parents to death.

It is required that students _____mobile phones at school, so seldom _____them using one.