JT-C系列机车信号主机内的高频开关电源块的作用是：将50V电压转换成（）电压，供板内电路使用。
参考答案：5V
颈段、胸上段食管癌的放射治疗中，由于颈胸部的肿瘤深度不同，为了使剂量尽可能的均匀，可用楔形板进行剂量调整，方向为尖朝脚，楔形板角度一般为（）
A.15°
B.30°
C.45°
D.60°
E.90°
According to the 80/20 rule, 80 percent of the problems are because of 20 percent of the causes. To facilitate finding out which are the most probable defects, and to evaluate nonconformities, you do a rank ordering of the defects using ______ .
A．statistical sampling
B．control chart
C．Pareto diagrams
D．inspection
