The final choice is the best answer. The main purpose of "Swarms of Terror" is to inform readers about an event from history that impacted farmers, whereas the main purpose of "Growing Up on a Farm" is to give readers a sense of what it's like to live on a farm today.
In the first choice, the first half of the answer (explain how the Rocky Mountain locust became extinct) works, but the second half of the answer (compare corn and soybean crops) does not.
The second and third answers do not best summarize the main purposes of these two texts. Only the final answer best summarizes the main purposes of "Sarms of Terror" and "Growing Up on a Farm."
