Nowadays senior high students are burdened with too many        ，which is making them less and less confident.

A．motivations

B．expectations

C．amusements

D．opportunities

空气制动阀作为大闸使用时，转换柱塞将单缓管至作用柱塞通路阻断，而沟通均衡风缸管至作用柱塞的通路，从而使该制动阀能控制（）的充风和排风。