题目

Don’t all speak at once! _____, please.A. Each at one timeB. one by one timeC. one for each timeD. One at a time   答案：D  one at a time 一次一个。  下列哪些地区发展种植业必须改造水分条件(    )A．青藏高原B．东南丘陵C．河西走廊D．河套平原