These boys haven't made up their minds

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没错,应该选A
when to start 做宾语
疑问词 + to do 相当于疑问词 + 从句
动词不定式（ to do ）可以和疑问词 what, which, who, whom, how, where, when 等连用,构成不定式短语.此结构具有名词的性质,在句中可充当主语、表语、宾语或同位语等.
一 . “疑问词 +to do ”在句中作主语.例如：
When to start isn't decided.
何时出发还没有决定.
注意：为了使句子均称,常使用形式主语 it ,把真正的主语“疑问词 +to do ”结构放在后面.例如：
How to do with the old car is a question = It's a question how to do with the old car.
如何处理这辆旧车是个问题.
二 . “疑问词 +to do ”在句中作表语.例如：
The question is where to get a puter.
问题是在哪里买计算机.
The question is what to do next.
问题是下一步该做什么.
三 . “疑问词 +to do ”在句中作宾语.
1. 在 tell, decide, teach, know, learn, wonder, discuss, explain, forget 等动词后,常用“疑问词 +to do ”作为它们的宾语.例如：
Sam is learning how to drive a car.
萨姆正在学习怎样驾驶一辆小汽车.
I don't know where to go.
我不知道该去哪里.
2. “疑问词 +to do ”在句中作宾语时,可与宾语从句相互转换.注意：没有“ why to do it ”的用法.例如：
Can you tell me how to get to the post office? = Can you tell me how I can get to the post office?
你能告诉我怎样才能到达邮局吗?
I don't know which book to choose. = I don't know which book I should choose.
我不知道该选择哪一本书.
3. “疑问词 +to do ”可作介词的宾语.例如：
Mr Li wants to write a work on how to swim.
李先生想写一部如何游泳的著作.
四 . “疑问词 +to do ”可在句中作同位语.例如：
The problem, who to do it, is being discussed.
至于谁去做这件事,这个问题正在讨论着.
make up one's mind to do sth
make up one's mind in doing sth. 下决心做某事======以下答案可供参考======供参考答案1:选A C 改为 when they started 也可以供参考答案2:C 为什么错了 ？” C. when they start “ 与”minds做同位语 将来时所以用 一般进行时start.供参考答案3:A对.一楼说的好.供参考答案4:A is right.供参考答案5:选A这道题考的是make up one's mind to do sth，由于意思需要，所以在make up 的宾补前加了when选C的话，要用when they will start