from March 15 to April 15, the change 6.3°
from April 15 to May 15, the change is 5°
from May 15 to June 15, the change is 4.3°
to find the average rate of change, we have to add the changes together, and then divide then by 3
6.3+5=11.3
11.3+4.3=15.6
15.6/3= 5.2
So your answer is C