—Must we keep the window all the time?—No you don’t have to.A.opening B.opened C. open

问题补充：

—Must we keep the window all the time?

—No, you don’t have to.

A.opening B.opened C. open

答案：

解析：我们必须一直保持窗户打开吗？不，不用。keep sth +adj,表示一种状态，后接形容词作为宾语补足语，故选C