“the graph shows the amount of water in a sink after several minutes”

First we can eliminate D because the line is going down so the coefficient for x has to be negative. 

to determine slope you look at ride over run(sideways). so the first point i see is (0,5) the next direct point is (2,2)
so the rise is -2 and the run is -3

with this information you can conclude your answer is C. y= -2/3+5

Yes. there is a constant ratio of number of calories to gram of protein.12/3=24/4=32/8=44/11=4