— When will they leave?

— They _____ very soon. New term _____ on September 1st. [ ]

A. do leave; begins

B. are leaving; begins

C. have left; will begin

D. leave, is beginning

元宵节是我国的传统节日，这一天小华全家团聚，煮汤圆以示庆贺．小华在帮妈妈煮汤圆时发现，生汤圆放入锅中由于浮力　 　重力而下沉，在煮的过程中，汤圆内部气体受热膨胀，浮力　 　（填“增大”“减小”或“不变”），致使浮力　 　重力而上浮．