TOEFL Reading: ETS-TOEFL阅读机经 - 1K45XQE61JD52773L$

According to paragraph 4, the solution of moving to other, previously occupied areas was effective only when A. the soil on the previously occupied land had been used for farming at least once B. the vegetation and nutrients of the previously occupied area had time to recover C. the reoccupations did not last a long time D. the reoccupied areas became more densely populated