Calculus 8th Edition

This function is presented on the graph $C$ and its level curves are on the graph II. This is because this function is periodical in the product of $xy$. This product grows most rapidly along the lines $x=y$ and $x=-y$ and we see that this function oscillates with greatest frequency along these lines. This is also reflected on the graph II because there the level curves get denser along the mentioned lines.