They elected him          captain of the team for         second time.

the; a

a; the

a; a

/; a

考查冠词的用法。句意：他们再次选他当队长。职位，官衔前面不加冠词，a second time表示“又一次，再次”；the second time表示第二次（顺序）。故选D项。

A study shows the students who are engaged in after-school  activities are happier than        who are not.

ones

those

these

them

考查代词。句意：一项研究表明，参加课外活动的学生比不参加课外活动的学生更开心。空格处代词与the students是对应关系，可以用the ones 或者those替代。“them”指代前面提到的复数名词，不能与the students形成对应关系，“these”一般不用定语从句修饰。故选B项。

There's no simple solution          this problem.

of

for

to 

about

考查介词。句意：这个问题没有什么简单的解决方法。此处是短语“

the solution to....的解决方法”，应用介词to。故选C项。

We’ll make the final decision on our scheme. ______ you change your mind, please inform us as soon as possible.

Should

Were

Would

Had

考查虚拟语气的省略。句意：我们将对我们的计划做最后决定。如果你改变主意，请尽快通知我们。根据语境，此处用虚拟语气，表示与将来相反，从句用should+动词原形，省略if，should提前，陈述句是：If you should change your mind……。故选A。