19. It’s dangerous to have the fire _______ at home all night long.
__________ good exercise.
I don’t know who invented __________ telescope, but I think it is __________ most useful invention.
Rarely _____________________ .
This is the only way we can imagine _______ the overuse of water in the students’ bathrooms.
Studies show that people are more _______ to suffer from back problems if they always sit before computer screens for a long time.