32. One trouble with U.S. agricultural workforce is_______. 

32.【答案】[D] the aging of immigrant farm workers

【解析】细节推理题，答案在文中第三段：They are also aging. At the start of this century, about one-third of crop workers were over the age of 35. Now, more than half are. 说明美国的农业劳动力在变老，并指出crop picking is hard on older bodies，与选项D吻合。选项A中的rising及illegal属于无中生有，选项B中的high mobility与定位段中的farm labors are more likely to be settled, rather than immigrating矛盾，选项C未涉及。