土坎梯田的施工工序包括（）。
A.施工定线
B.田坎清基
C.修筑田坎
D.保留表土
E.修平田面
参考答案：A, B, C, D, E
世界上只有中国文化不曾中断而一直延续至今，形成一个________ 、________ 而又兼容并蓄的开放体系。
He was about to jump into the river ________ his mother stopped him. [ ]
A. while
B. when
C. as
D. before
