设3阶方阵A、B的行列式A=2，B=-3，则-ATB2等于（）．
A.18
B.-6
C.-18
D.-9
参考答案：C
The boy failed four times, but he said he would try ___________.
A．a five time
B．a fifth time
C．the five time
D．the fifth time
细胞外基质(extracellular matrix)
