Professor Adeyemi is examining well-being after retirement in a city, and it is important to have excellent external validity. Professor Adeyemi selects two random numbers, 4 and 6, and gets a list of all of the retired people in the city. If Professor Adeyemi obtains the sample by starting with the 4th person on the list and selecting every 6th person, which of the following sampling techniques is most likely being used?

a. systematic sampling
b. stratified random sampling
c. quota sampling
d. cluster sampling