The first term is 8 as this is the y coordinate of the point when x = 1. The second term is 4 (since y = 4 when x = 2) etc etc
The sequence of terms is: 8, 4, 2, 1, ...
Each time we're dividing by 2, or multiplying by 1/2. This is why r = 1/2
Put this together and we have this geometric nth term formula an = 8*(1/2)^(n-1)
This means the answer could be A or it could be C. However, we can rule out choice C because the rate of change is not positive. Draw a line through (1,8) and (3,2) and you'll see this line has a negative slope. So the rate of change is negative.
