题目

—How was   ________  they discovered the entrance to the underground palace? —Totally by chance. A．he that         B．it that     C．it when  D．he which   答案：B   石块A自塔顶从静止开始自由落下s1时，石块B从离塔顶s2处从静止开始自由落下，两石块同时落地，.则塔高为                                              （     ）     A．s1+s2 B．   C．   D．