It’s quite normal to ten percent of the cost of a building for a period of six months or s

问题补充：

It’s quite normal to ten percent of the cost of a building for a period of six months or so, in case faults are found in it.

A. keep up

B. keep away

C. keep off

D. keep back

答案：

A. keep up

B. keep away

C. keep off

(答案→)D. keep back

解析：短语辨析. A. 保持;继续 B. 防范;不接近 C.让开 D.留下;阻止.句意：扣留大约十分之一的建筑成本六个月左右的一段时间是很正常的事情，以防万一在里面发现什么问题。结合语境可知选D。