A list is made of all possible 2-digit whole numbers that result from using only the digits 1,3,5, and 7 in both the ones and tens place, with duplications allowed. How many whole numbers in the list are prime numbers?

A.6
B.7
C.8
D.9
E.10​

Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour from 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?