Which of the following is a complex sentence?

A. I would like to visit the Kinshasa, but I understand the city is unsafe for foreigners.
B. Check out the editorial page in today's paper and you'll notice the piece by George Will.
C. I simply cannot make heads or tails of rap lyrics, but I suppose I'm just too old to understand them.
D. As Jake was reading the email from June yet again, the flight attendant coughed gently to get his attention.