challenge: What is a challenge faced by proposals that modify gravity to solve the cosmological constant problem?
  
  A. They are inconsistent with the results of observations and experiments that confirm the existence of dark matter.
  B. They are inconsistent with the results of observations and experiments that have been extremely consistent with general relativity and the ΛCDM model.
  C. They are inconsistent with the results of observations and experiments that have been inconsistent with the Big Bang theory.
  D. They are inconsistent with the results of observations and experiments that confirm the existence of dark energy.
  Answer: 