Which of these items did the united states export in the late 19th century apex?

Although during the 19th century the United States were exporting many different things, one of the most popular commodities was farming equipment. In colonial America, over 90% of the population used a form of agriculture to make their money, and by the start of the 20th century the number of farms had grown to well over 6 million, scattered throughout the country.

4.a depression is the answer

These islands/chinampas then provided rich soil for crops with easy access to water. i hope this is correct, im so sorry if its not !