∵-a3有意义
∴a≤0
∴原式=-a-a．
故选B．
地基土的强度破坏是剪切破坏，而不是受压破坏。
GM cannot be used as an indicator of stability at all angles of inclination because（）.
A.M is not fixed at large angle
B.there is no M at large angles
C.G is not fixed at large angles
D.there is no G at large angles
