背斜在构造图的层面上表现为中心等高线值（），两翼等高线值（）。
参考答案：高；低；
— Each of the students, working hard at their lessons, ________ the book.
— So have I. [ ]
A. is reading
B. has read
C. reading
D. reads
3G有预付费客户吗。
have I. [ ]