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 quit 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 the clerk providing the offer did not know the details of the offer. After the$$ program, clients were followed for six months to see if they had indeed quit smoking.