It is B. the differences are added together
Week 15 they'll both weigh 181 lbs
Since the surface is the top part of a sphere centred on the origin, the directions of steepest descent are any vector originating from the origin (0,0).
At (5,5), the direction would be dictated by the line joining (5,5) and the origin (0,0), i.e. along <5,5> or equivalently along <1,1>.
You can also go through vector calculus to arrive at the same result.