What should the writer add to the end of the sentence in order to create a complex sentence?

A. in the morning before english and science.

B. but we also study math and geography

C. if we have free time in the afternoon

D. and we learn the history of Ancient Greece

I believe the answer is C, im not 100%. but i know its not B or D

The answer is letter B. but we also study math and geometry. This independent clause will make a complex sentence to the other independent clause. The word "but" serves as a contradictory idea and makes a clue of the the previous idea which makes a complex sentence.