We can write the cost function as :

cost(x) = 120x + 36(100 - x)

= 120x + 3600 - 36x

= 84x + 3600

So we can see that it is a strictly increasing function hence x = 0 will give minimum cost i.e. at town B.

If we write w.r.t. town A , we get :

cost(x) = 36x + 120(100 - x)

= 12000 - 84x

So we can see that it is a strictly decreasing function hence x = 100 will give minimum cost i.e. at town B.

So , in either case we get the town B as the place where we get minimum cost.

**Hence D) should be correct option.**