Swimmers must be very strong in order to build up speed because of the high resistance (阻力)of the water. Runners,__________, must struggle against the heavy resistance of the air and ground.
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.
The train ____ at present speed until it reaches Beijing.