I didn’t quite follow him because he explained the theory _____that is too abstract for me.
Tom as well as his friends ____ to visit our school soon after they arrive in Changsha.
By the time you ____ in, I ____ two letters.
To ____ your dream of becoming a scientist, you must work hard from now on.
I ____ go to bed ____ the telephone rang.
You may ask the teacher for help ____ difficulty.