Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour after the time that she first checked her outdoor thermometer. By 6 a.m., the temperature had fallen 21 degrees. Which expression can you use to find how many hours earlier she had first checked the thermometer?
a Negative 21 divided by negative 1.4
b Negative 1.4 divided by negative 21
c Negative 21 divided by 1.4
d 21 divided by negative 1.4