WHich equation models the number of leaves y along x in. of vine

There are 2 leaves along 3 in. of an ivy vine. There are 14 leaves along 15 in. of the same vine. Which equation models the number of leaves y along x in. of vine?

a. y=X+1
b. y=-x+1
c. y=-x-1
d. y=x-1

I'm not sure why (a) and (b) are the same and (c) and (d) are the same, but the answer is

y = x - 1

There is one less leaf than inches in the vine.

d