21. To many teenagers the cellphone is not only _____useful tool but also ____ way to have fun and be cool.
A red sky in the morning ______ to be a sign of bad weather.
Hundreds of jobs _______ if the factory closes.
“Look! Everything here is under construction.” “What is the small building that ______for?”
He kept a little notebook, in which ______ the names and addresses of his friends.
“How about the dishes, Dear?” “The beef didn’t taste very good. It ______ too long.”