According to structure, which kind of sentence does the following sentence belong to?I looked for Mary and Samantha at the bus station, but I did not see them, because they had left before I arrived at the station.

According to structure, which kind of sentence does the following sentence belong to?I looked for Mary and Samantha at the bus station, but I did not see them, because they had left before I arrived at the station.
A.Simple
B.Compound
C.Complex
D.Compound-complex
正确答案：Compound-complex