脊索动物门的下列特征中，哪一种也存在于成年的被囊动物（尾索动物亚门）中？（） ["有脊索","有鳃裂","有尾","有背侧管状神经系统"]
杆塔按用途分为哪几类？各有何特点？
If you increase the clearance between a valve stem and rocker arm, which of the listed condition will occur?（） ["valve will open later","valves will close later","amount of fuel injected will be increased","amount of fuel injected will be decreased"]
A venetian vent or rice ventilator is used in stowage of which cargo?（）. ["Bagged cargo","Bulk grain","Bulk rice","Refrigerated good"]
如何保证电能质量的频率指标？
开关操动机构主要有哪几种？有何优缺点？
