梯形的上、下底的长分别是8厘米和6厘米，则此梯形的中位线的长为（ ）厘米．
7
With a lot of difficult problems _______, the manager felt worried all the time.
A．settled
B．to be settled
C．settling
D．to settle
为什么合成纤维后处理过程中，要进行热定型？
