单项选择题 In the compression refrigeration system, the refrigerant liquid with low temperature and high pressure changes into the liquid with low temperature and low pressure through the_____.

多项选择题 下弦中央节点的构造与（       ）和（     ）有关 A、两斜腹杆 B、长细比 C、下弦杆的交角大小 D、自重

单项选择题 有计划地将抗菌药物分期分批交替使用，其原因是