Which of the following sentence is a complex sentence ?（ ）

A、Mary and Samantha realized that Joe was waiting at the train station after they left on the bus.
B、I looked for Mary and Samantha at the bus station, but they arrived at the station before noon and left on the bus before I arrived.
C、I looked for Mary and Samantha at the bus station.
D、Mary and Samantha arrived at the bus station before noon, and they left on the bus before I arrived.