托福听力official32 Why does the author mention a plowed field and a construction site in the passage? 答案解析

用plowed field and a construction site先定位细节出处整段最后一句话，那么这个细节所支撑的目的句要么在它前1.2句，要么就是整段的主旨句了。很明显，其所突出的目的句就是前一句话，结合这两句话来进行选择：For a given rate of invasion, colonization of a moist, fertile site is likely to be much more rapid than that of a dry, infertile site because of poor survival on the latter.这句话的内容只要是说，入侵肥沃的土地比那些干涸的、不肥沃的土地要来的快很多。这就和C选项呼应。