문제 내용: 다음 글의 내용과 일치하는 것은?
선택지 1: A churchyard in Iowa is higher than the surrounding farmland.
선택지 2: Iowa's agricultural productivity has accelerated its soil formation.
선택지 3: The rate of soil formation in farmlands is faster than that of soil erosion.
선택지 4: Iowa has maintained its top soil in the last 150 years.