汇集—____ 擎着—____ 耸立—____
—______ delicious the beef is! Thank you for inviting me to dinner.—I'm glad you like it.
把蜡烛放在距离凸透镜50cm处,在透镜另一侧的光屏上观察到倒立、缩小的清晰像.那么凸透镜的焦距不可能是( )