Ⅰ.单句语法填空
1．But in fact sports and games can be of great ________(valuable)，especially to people who work with their brains most of the day.
2．The teacher's explanation clarify the ________(puzzle)problem.
3．She has an ________(annoy)habit of biting her fingernails.
4．It was not until 12 o'clock last night ________ my father came home.
