When we are young, questions of _______we want to be when we grow up are common

John, you must get your room ________ after breakfast[ ]

Qian Xuesen tried many times _________ he found thesolution to the puzzle about space technology

Nowadays people sometimes separate their waste to make it easier for it ________.[ ]

you a teacher— No, but I ____. I worked in a middleschool for three years.