Suppose researchers are interested in how vouchers for health check-ups affect subsequent health outcomes. They want to randomize the provision of vouchers (treatment) at the individual level. Which of the following is not a failure to randomize: (a) Individuals that received the vouchers tended to drop out of the study. (b) The research assistants who worked at the health clinics gave vouchers to individuals that appeared to be sick and/or poor. (c) The researchers gave the first half of n clients who walked into the health clinic each day the vouchers and the remaining half of n clients who walked into the health clinic later in the day did not receive the vouchers. (d) By chance, individuals who received the vouchers tended to be healthier at baseline than individuals who did not receive the vouchers.