challenge: What is the ring of modular forms of Γ composed of?
  
  A. The direct sum of modular forms of weight k for k < 0
  B. The quotient of modular forms of weight k for k ≥ 0
  C. The direct sum of modular forms of weight k for k ≥ 0
  D. The tensor product of modular forms of weight k for k ≥ 0
  Answer: 