Which of the following sentence is a simple sentence ?（ ）
选项：

A:Mary and Samantha left on the bus before I arrived, so I did not see them at the bus station.
B:Mary and Samantha arrived at the bus station early but waited until noon for the bus.
C:Joe waited for the train, but the train was late.
D:While he waited at the train station, Joe realized that the train was late.