Although there is no context provided, time in history, or further reference, we can say the following.
The correct answer is false.
It is false American farmers grew enough to sell within the USA but did not trade with other nations.
Depending on the context or moment in history that you do not mention, but in general terms. American farmers earn a fair amount of money due to exports. Of course, they trade their products in teh United States. But exportations represent a big business for farmers and food companies.
There have been times when the US government has asked farmers to stop producing because there is an overproduction of crops. The US federal government has paid American farmers to stop production until the market stabilizes.