A cup of boiling water ($212$\textsuperscript{◦}F) is placed to cool in a room whose temper- ature remains constant at $68$\textsuperscript{◦}F. Suppose the difference between the water temperature and the room temperature is halved every $5$ minutes. What is the water temperature, in degrees Fahrenheit, after $15$ minutes? 
选项为:A:$$77$$  B:$$86$$  C:$$92$$  D:$$98$$  E:$$104$$  请回答其中一个选项.