For the following experiment, indicate whether it was single-blind, double-blind, or not blinded at all. Explain your reasoning.

Does a “stop smoking” program work better if it costs more? Smokers responding to an advertisement offering to help them stop smoking were randomly offered a program costing $100 or the same program costing $250. The offer was made individually to each client by presenting a sealed envelope so that the clerk providing the offer wouldn’t know the offer’s details. At the end of the program (a course and films along with diet and smoking cessation aids), clients were followed for six months to see if they had indeed quit smoking.