"Railroads were the key reason for US economic growth 1865-1890" Assess the validity of this view.

There are many ways to answer this question, but I will illustrate one way of doing it, then explain how to apply this method to answer it differently. To gain marks, it's normally easier to disagree with the question, as you ended up weighing up multiple views, allowing for more evaluation. For the introduction, I would write that the truth of the matter is that exploitation was the driving factor behind US economic growth in the era, and that 3 key areas of exploitation were railroads, natural resources, and the labour force. Then, for the middle of the essay, I would do a paragraph for each point, using facts and statistics to indicate how each one was important for growth, and how they were exploited. Finally, as a conclusion, I would link all of the 3 ideas together via exploitation, and argue that the key reason is the biggest driving force, and exploitation was the underlying factor for growth, hence the biggest reason, and hence the key reason.

To apply this to any other "X was the key reason for..." question, simply find 3 different key reasons and identify a common link. Then, allow the link to become the key reason. as you end up proving the merit of each reason, but limiting it by stating it's part of a bigger picture, gaining marks for having a balanced evalutation and a sustained argument.