题目

There were some chairs left over _____ everyone had sat down.    A. when        B. until         C. that       D. where 答案：A  解析: 从句中had sat down动作发生在主句were之前，可以填after，所以选A。人类新个体的产生，要经历 细胞的结合，这一过程主要是靠 完成的．