图12-2-9（a）表示一列简谐波在介质中传播时，某一质点a的振动图象，请你在（b）图中作出这列简谐波在t=0时刻的波形图（质点a画在坐标原点上）. 图12
—How long have you known the doctor ? — he helped me three years ago.A．When B．Before C．BecauseD．Since
Some researchers, parents and teachers have suggested that high-school classes begin later in the morning to accommodate (适应) teens’ needs for more sleep. 31 However, in the m
能促进细胞的分裂和生长,使枝叶长得繁茂的无机盐是 A．含氮的无机盐 B．含磷的无机盐 C．含钾的无机盐
放牧强度可影响人工草地的物质循环和能量流动。图1是亚热带某人工草地在不同放牧强度下牧草净光合产量（净光合产量=光合作用合成量—呼吸作用消
Every eye ________________（注视） the new girl when she entered the classroom. （fix）
She saw a brook with red flowers and green grass on all sides. A B C D
More than one-third of the Chinese in the United States live in California, _____ in San Francisco. A. previously B. predominantly C practically D. permanently
：
，则 
B. 
D. 
