challenge: What condition must be met for a random variable Y to be considered mean independent of random variable X?
  
  A. The conditional mean of Y is greater than its unconditional mean for all x
  B. The conditional mean of Y equals its unconditional mean for all x where the probability density of X at x is not zero
  C. The probability density of Y at x is not zero
  D. The probability density of X at x is zero
  Answer: 