r = 6 is one possible value of r
In order for r to equal 6, it must be the case that the line passes through the origin (0, 0)
In this case, the slope between (0, 0) and (3, 6) is 2
If r = 6, then Quantity A is greater


Now let's see what happens if the line does not pass through the origin
Let's say the line passes through the point (0, -6)
In this case, we need the slope from (0, -6) to (3, r) to be 2
Slope = (r - -6)/(3 - 0) = 2
Simplify: (r + 6)/3 = 2
Solve: r = 0
If r = 0, then Quantity B is greater


Answer: D

Ans is D, checked
End equation is r-c=6