题目

Everyone in our class was invited (take) part in that activity. 答案：【1】to take将一支点燃的蜡烛放在凸透镜前24 cm处，在凸透镜另一侧的光屏上得到一个清晰等大的像。则该凸透镜的焦距为（ ）A. 10cm B. 12cm C. 20cm D. 30cm