引起前置胎盘病因有（）
A.前次剖宫产
B.多次人工流产
C.盆腔炎
D.吸烟
E.服用可卡因
参考答案：A, B, D, E
She made a few mistakes playing the violin, but you should make ____ for her since she’s only been playing for a year.
A．allowances
B．promises
C．room
D．way
A．t=0.2s时，CD间F点的位移最大
B．t=0.2s时，CD间E、F、G三点的位移最大
C．t=0.5s时，CD间只有F点的位移最大
D．t=0.5s时，CD间E、G两点的位移最大
