汉译英

汉译英
1.足够的锻炼才能确保足够的健康.
Only____ ______ can keep us_____ _____
2.她从不在别人面前表露感情.
She never ______ herfeelings before _______
3.你知道这场大火是由什么引起的吗?
______ do you know _______ the big fire?
4.还有谁没交作业?
_______ ________ haven't ______in their exercise books?