The following sentence contains either a single error or no error at all. If the sentence contains an error, select the one underlined part that must be changed to make the sentence correct.If the sentence contains no error, select choice E.

　　When people gave up the hunter-gatherer way of life and began to cultivate the soil and grow their food, they often became less mobile, built more substantial residences, and they developed more effective means of storage. No error

　　(A) When

　　(B) became less mobile

　　(C) they developed

　　(D) more effective means

　　(E) No error

　　Explanation

　　The error in this sentence occurs at (C), where there is a flaw in parallelism. The pronoun "they" should be removed so that the sentence contains three parallel verbs ("became," "built," and "developed").