Consider the case in which pupils in school are eligible for a free healthy lunch including 2 pieces of fruit in case parental income is less than 20,000 Euros. John is interested in the effect of fruit intake on the health of pupils. What is a direct threat against the validity of a Regression Discontinuity Design (RDD) in this case? a. A very low fruit intake among those with very low incomes b. Families who keep silent about part of their income to be eligible for free lunch c. A direct effect of parental income on health of the children d. Non-linearity of the relationship between fruit intake and income of the parents