Which of the following is a complex sentence?

AI simply cannot make heads or tails of rap lyrics, but I suppose I'm just too old to understand them.

BAs Jake was reading the letter from June yet again, the flight attendant coughed gently to get his attention.

CCheck out the editorial page in today's paper and you'll notice the piece by Oscar Lewis.

DI would like to visit the Kinshasa, but I understand the city is unsafe for foreigners.