It has been announced that students         remain in their seats until all the papers have been collected.

A．can

B．will

C．shall

D．may

功效牙膏的保健功能取决于（）。

A、牙膏中含有高效的成分

B、各种有效成分科学配比

C、牙膏中特定的功效成分属于天然提取物

D、牙膏功效符合口腔自然的生理和节律