选择题：材料力学研究的内力是构件各部分的相互作用力

A short sentence after a series of(一系列) long sentences can have a dramatic _____

企业对外销售产品时，由于产品经营和品种不符合规定要求，而适当降价，称作A A.销售退回B B.价格折扣C C.销售折让D D.销售折扣

构造主义的目标是 A.了解决定行为的目的B.把意识分解成最基本的成分C.研究人们的意识D.研究人们的意识

The ship's generator broke down, and the pumps had to be operated instead of

Across the continent of South America （）the Amazon River. A.lie B.lies C.isly