Final answer:
The sentence that supports the claim in paragraph one is about the complex structures of bathing facilities and the presence of rooms for transitioning between different temperatures.
Step-by-step explanation:
The sentence from the passage that best supports the claim in paragraph one that George Washington founded the country's first spa is:
Dr. Zucker: These were really complex structures, these bathing facilities. There were the baths themselves. Some were cold, some were hot. There were rooms for transition from one temperature to another. There were places where one could exercise and this sculpture makes perfect sense in this environment. This is a place where you would go to work out, where you would go to exercise. You could look at this wildly muscular figure and have a bit of a goal.