Answer:
Europeans generally used a one-crop economy in their colonies, for they are trying to generate wealth, instead of looking after the economies of the colonies. Most of the time, the crop is a "cash crop" or a crop that is worth a lot when needed, but usually cannot be eaten or used as food. They can include Tobacco, Cotton, etc.
However, the usage of only producing one product would lead to a large dependancy on it, and the economy of the certain country would depend on the demand as well as the price of the product. The higher the demand, the better the price, which leads to a better GDP. Of course, this may change on the whim, which makes an extremely unstable economy.
In today's world, only one one-resource economy works, and that is oil. Oil is used for a lot of things, including creating gasoline, diesel, etc, and is used in many modern appliances. However, with the world soon seeing the disastrous affects of the usage of fossil fuels, they are slowly transitioning to renewable resources, which may soon put the "oil kings" out of business, and propelling them back into a third-world status.
In the end, the Europeans used one crop economies in their colonies to generate quick cash, and to keep the economies of the colonies small, which may lead to them having trouble with uprisings.
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