题目

By the time the teacher _______ back, we ________ the thorough cleaning. A . came, have finished B . comes, have finished C . will come, will have come D . comes, will have finished 答案：D 西周时“一人（踏）耒而耕，不过十亩”；战国时“一夫挟五口，有田百亩”。引起这一变化的根本原因是 [　　] A．公田变为私田 B．铁器牛耕的推广 C．国家走向统一 D．中国原始的自然崇拜