题目

The lecture given by the professor from Nanjing University covered the ________of audio devices，beginning with Thomas Edison's first recording. A．change  B．revolution C．reform  D．evolution 答案：D 考查词义辨析。change“改变”；revolution“革命”；reform“改革”；evolution“演变，逐步发展”。此处指“音频设备的发展(过程)”。已知△ABC为等腰三角形，PA⊥平面ABC，AB=AC=5，PA=BC=53，求：（1）点P到直线BC的距离；（2）二面角B-PA-C的大小．