— May we leave the classroom now?
— No, you ________. You ________ to leave until the bell rings.
A：mustn't; are allowed
B：don't have to; are supposed
C：needn't; aren't allowed
D：can't; aren't supposed
下列函数是指数函数的是（ ）
激光可以做成很细的光束，这是利用了激光的____的性质，激光是____（纵/横）波．在光导纤维中传递信息要比在真空中____（快/慢）．
