—I didn’t tell him the news.—Oh, you ought _______.

----May we leave the classroom now?----No, you         . You           to leave until the bell rings.

—Lily,______ do you study for a math best? —By asking my teacher for help.

—Can I wear any clothes I like to school?—No, you can’t. You ___ wear a uniform.

Trees ____ fight air pollution．They are natural air conditioners．