解方程：3x-21=15-5x-20．
移项，得3x+5x=15-20+21，…2分
合并，得 8x=16，…4分
系数化成1，得x=2． …6分
在OSI/RM中，（）实现数据压缩功能。
A.应用层
B.表示层
C.会话层
D.网络层
– Do you know _____ this time yesterday?
-- Sorry, I don’t know.
A. what she is doing B. what was she doing C. what she was doing
