Which of the following is a complex sentence?

a. as jake was reading the letter from june yet again, the flight attendant coughed gently to get his attention.
b. i simply cannot make heads or tails of rap lyrics, but i suppose i'm just too old to understand them.
c. check out the editorial page in today's paper and you'll notice the piece by oscar lewis.
d. i would like to visit the kinshasa, but i understand the city is unsafe for foreigners.