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一、选择题1.科技中学的物理兴趣小组在探究凸透镜成像规律时,用4块透镜分别进行实验。他们把蜡烛均放在离透镜20cm处,观察到的现象是:用透镜甲可在光屏上得到倒立、缩小的像;用透镜乙可观察到正立、放大的像;用透镜丙可在光屏上得到倒立、放大的像;用透镜丁可在光屏上观察到倒立、等大的像。那么,焦距小于20cm的凸透镜不可能是( )。
First, multiple-choice questions 1. The Physics Interest Group of the science and technology middle school conducts experiments with four lenses while exploring the law of convex lens imaging. They placed the candles at 20 cm from the lens. The observed phenomena were: using lens A to obtain an inverted and reduced image on the screen; using Lens B to observe an erect, enlarged image; using lens C in Inverted and magnified images are obtained on the light screen; invertible, equal-sized images can be observed on the light screen with the lens. Then, the convex lens with a focal length of less than 20cm cannot be ().