Let's learn to use the problem we are facing         a stepping-stone to future success.

to

for

as

by

考查介词词义辨析。句意：让我们学会把我们所面对的问题作为通向未来成功的手段来利用。A.to朝，对 ；B.for 为了，对于；C.as 作为；D.by通过，凭借。根据句意可知，此处“作为”符合语境。故选C项。

Mary and her friend          a plan and gathered forty people from my company to help the homeless people at the Christmas party.

came up with

put up with

caught up with

kept up with

考查动词短语辨析。句意：玛丽和她的朋友想出了一个计划，在圣诞节聚会上，从我的公司聚集了四十几个人来帮助无家可归的人。A. came up with 提出；B. put up with 忍耐；C. caught up with追上，赶上；D.kept up with 跟上。下文“聚集人帮助无家可归的人”是想出的计划。故选A项。

Though ____ the dangers of smoking to their health, smokers still find it hard to give up smoking.

aware of

are aware of

be aware of

考查状语从句的省略：句意：虽然很多经常吸烟的人已经意识到抽烟对他们健康造成了很严重的后果，但是他们仍然发现很难戒烟。Though引导的让步状语从句是省略句，完整的句子是：though smokers are aware of the dangers of smoking to their health。当主从句的主语一致的时候，可以省略从句的主语和be动词，选B

Great amounts of water            poured into the river from the factories two years ago.

was

were

has been

have been

考查时态和主谓一致。句意：两年前，工厂将大量废水排入河中。根据two years ago 可知，句子使用一般过去时。....amounts of修饰不可数名词作主语时，谓语动词用复数形式，即谓语动词与amounts保持数的一致。故选B项。