Mr. Smith thought of himself as _______,and this accident had ________ effect on his life.

A．a failure; a

B．failure; an

C．a failure; an

D．failure; the

如图所示，有一根长L=5 m的铁链悬挂在某楼顶上，楼中有一窗口，窗口上沿离铁链的悬点H=25 m，当铁链从静止开始下落后始终保持竖直，不计空气阻力，g取10 m/s²，求：

（1）铁链的下端A下落到窗口上沿B时，铁链的速度大小；

（2）接着铁链经过整个窗口时用了t=0.3 s的时间。窗口的高度h为多少？