1 The valve is closed to keep system fluid. 主语：the valve ?谓语: is closed宾语：–；目的状语：to keep system fluid.
A.1
正确答案是A
1 The valve is closed to keep system fluid. 主语：the valve ?谓语: is closed宾语：–；目的状语：to keep system fluid.
A.1
正确答案是A