An adolescent with type 1 diabetes returns for a follow-up visit after his annual checkup last week. You note that his serum glucose is elevated, and his glycosylated hemoglobin (hemoglobin A1C) is 16.7%. This finding suggests poor control of his diabetes over at least which of the following time periods?
a) Past week
b) Past 2 weeks
c) Past 3 months
d) Past 6 months