American fathers ______ more than twice the amount of time with their children than they used to in years past, due to a sweeping new trend called "unemployment".

A．were spending

B．spent

C．are spending

D．have spent

现代飞机上大量使用复合材料蜂窝结构，其主要优点之一是（）。

A.能够较好的抵抗声音的振动

B.不需要检查

C.因为它很重，所以比较坚固

D.比较耐火