I maintained that temperature for 10 minutes ( according to McGee 's instructions ) before moving the pan to a cold water bath and lowering the temperature to 115 degrees , at which point I mixed in 3 tablespoons of our favorite commercial yogurt from Seven Stars Farm . I poured the mix into a warm quart jar and placed the jar in a cooler with a heavy pot full of hot water . I placed an instant - read thermometer in the cooler , aiming to maintain a temperature of about 115 degrees . Turned out I did n't need to do anything further .

Question with options to choose from: Why did they place a thermostat in the cooler ?
Choices:
(a). It was important that they had the temperature perfect ..
(b). They were testing out different temperatures for the cooler ..
(c). None of the above choices ..
(d). They were unsure what temperature to set the cooler at ..
(a).