Let $V$ be the volume of the solid that lies under the graph of $f(x, y)=\sqrt{52-x^2-y^2}$ and above the rectangle given by $2 \leqslant x \leqslant 4,2 \leqslant y \leqslant 6$. We use the lines $x=3$ and $y=4$ to divide $R$ into subrectangles. Let $L$ and $U$ be the Riemann sums computed using lower left corners and upper right corners, respectively. Without calculating the numbers $V, L$, and $U$, arrange them in increasing order and explain your reasoning.