Which sentence from the passage is an example of a compound sentence?

Question

A.
However, few people know about Sir Arthur Conan Doyle, the man who created Sherlock Holmes.
B.
By the time he returned to England, he was very ill.
C.
After medical school, Doyle worked as a medical officer aboard a ship called the Mayumba, but he resigned after one trip to Africa.
D.
He graduated from the boarding school at the age of 17 and went on to study medicine at the University of Edinburgh.

Answer 1

D. He graduated from the boarding school at the age of 17 and went on to study medicine at the University of Edinburgh.