若管道腐蚀深度大于0.5倍壁厚，其腐蚀级别可判定为（）。
A.轻
B.重
C.极重
D.危险
参考答案：D
All the books donated by the students from urban schools are said _____ to the students in rural areas
the other day. [ ]
A. to be sent
B. being sent
C. having been sent
D. to have been sent
“八百里秦川”是指（ ）
A．黄土高原
B．陕北高原
C．汉江谷地
D．渭河平原
