challenge: What is a significant challenge in predicting the orbits of comets, according to Syuichi Nakano's area of study?
  
  A. Only the comet's size and shape
  B. Only perturbations from the planets
  C. Both perturbations from the planets and non-gravitational forces due to the release of gaseous material
  D. Only non-gravitational forces due to the release of gaseous material
  Answer: 