题目

 People often find themselves caught in such a _____ as “whether…or not…”.    A. dilemma                     B. endeavor           C. fault                  D. survival 答案：A如图所示，两条入射光线（或延长线）分别过凹透镜的光心O和焦点F，分别画出经过凹透镜折射后的光线．