Which of the following is a complex sentence?
a. I simply cannot make heads or tails of rap lyrics, but I suppose I'm just too old to understand them.
b. Check out the editorial page in today's paper and you'll notice the piece by Oscar Lewis.
c. I would like to visit the Kinshasa, but I understand the city is unsafe for foreigners.
d. As Jake was reading the letter from June yet again, the flight attendant coughed gently to get his attention.