题目

 A new _____ bus service to Tianjin Airport started to operate two months ago.    A. normal                                        B. usual                      C. regular                D. common 答案：C如图，玻璃棱镜的截面为等腰三角形，顶角α为30°．一束光线垂直于ab面射入棱镜，又从ac面射出．出射光线与入射光线之间的夹角为30°，则此棱镜材料的折射率是 A．　　B．      C．　　  D．