—Have you seen your friend off ?
—Yes. I watched the train leaving the station it disappeared from sight.
A. unless
B. as if
C. so that
D. until
网友回答
A. unless
B. as if
C. so that
(答案→)D. until
解析：考查词和词组意思，unless意思“除非”， as if意思“好像”，so that意思“以至于”，until意思“直到……”，句子意思“我看卓火车离开车站直到从我的眼前消失”，所以选D