Most probably it will be another 7 hours _____ you can have next meal, so each of you must have

your stomach full.[ ]

A. since            

B. when          

C. after            

D. before

吸声材料的吸声系数应该（）。

A、大于0.2

B、小于0，2

C、大于0.I

D、大于0.2并小于0.5