O____o oh my god!
Let f,S and Dr be as above.
Define g:S->R as g(x,y) = f(x,y) and let J be the set of suits Beth can wear.
Note that J is a subset of S. Define the suit rosala picked as Jr=(10,10).
Note that Jr = Dr. Then given that Dr attains a maximum on f(x,y) and is therefore the best dress of set D, Jr must attain a maximum on g(x,y) = f(x,y) and is therefore the best suit of set J.
Hence Jr is the suit Beth likes most.
phieuw... 