题目

24. ______ life began on earth is one of the biggest puzzles that scientist found hard ______. A. How; solving                                 B. When; to be solved C. How; to solve                                D. Which; to solve 答案：B某同学进行“观察叶片的结构“实验，所看到的菠菜叶横切面如图，下列说法错误的是（　　）A．图中[3]是叶肉B．叶片与外界进行气体交换的“门户”是图中的[4]气孔C．图中[1]是下表皮D．气孔的开闭由[5]保卫细胞控制