A case-control study was set up in a large town in India to measure the efficacy of measles vaccination. The study population consisted of all children aged between 5 and 10 years who were registered at a local government medical clinic. Cases were 500 children who were recently diagnosed with measles at the clinic. Measles vaccination status was ascertained by asking the parents if a vaccination had been received. Controls were selected from the children attending two schools in that area. What problem might this choice of control population lead to?

a. Non-differential misclassification bias

b. Measurement bias

c. Differential misclassification bias

d. Selection bias