—Have you been to Australia?

—Yes, only once. I ____ there only for seven days. [ ]

A. have stayed    

B. was staying    

C. stayed          

D. had stayed

在河里游泳时最怕遇到漩涡．当人进入漩涡边沿后，往往不由自主地被吸入漩涡中心，造成事故．这是因为相对于漩涡边沿，漩涡中心流速         ，压力较        ，从而形成指向漩涡中心的压力差，把人吸进去．