The answer is A.the freezing point of water and the boiling point of water.
Fahrenheit based his scale on the coldest temp he could obtain, a mixture of snow and salt. He figured that boiling was somehow opposite freezing in the same way that one side of a circle is opposite the other, so he put the two 180 degrees apart. By that scale, he determined that the coldest temperature he could obtain, a mixture of snow and salt, was 32 degrees below the freezing point of water, and he set that as his zero point. Yes, scientists get goofy like that sometimes.
That means a centigrade degree is 1.8 times as big as a Fahrenheit degree, and Fahrenheit is offset from Celsius by 32 degrees. So to convert from C to F you use F = 1.8C + 32 and to go the other way you use C = (F - 32)/1.8 Both scales are equal at -40 degrees.
