there are only 20 minutes left before the train left I felt uneasy in the taxi to the rail

问题补充：

there are only 20 minutes left before the train left，I felt uneasy in the taxi to the railway station.

A. That

B. When

C. Which

D. Because

答案：

A. That

B. When

C. Which

(答案→)D. Because

解析：because引导原因状语从句。因为“只剩二十分钟”，所以才“感到不安”，合乎逻辑。