.It is reported that two schools,_______fell into ruins in the earthquake, are being rebuilt and will be open next term.

A．they both

B．both of which

C．which both

D．both of them

核心竞争力是伴随知识经济而产生的一个新概念。普拉哈拉德与哈默尔在描述核心竞争力时，提出了非常形象的“树型”理论，他们将企业比作棵大树。核心竞争力就是这棵大树的( )。

A．树干
B．叶子
C．果实
D．根系