Read the excerpt from “The Future of Money.”
You can also earn bitcoins by doing certain kinds of work, just as you can earn traditional money by doing traditional work. What work do people do to earn bitcoin? Computer experts can “mine” bitcoin by solving complex math problems related to the blockchain. Traditional work might be selling lemonade, or cutting people’s grass, or washing cars.

Based on the context of the paragraph, what is the most likely meaning of the word complex?
A. easy to identify
B. related to money
C. difficult to solve
D. regularly occurring