The region R is bounded by y=3, y=0, x=0, and x=2. Two solids share R as a common base. Solid S1 has cross sections perpendicular to the x-axis that are squares, while solid S2 has cross sections perpendicular to the x-axis that are semicircles whose diameters lie in R. Without performing any integration, determine which solid has the greater volume and briefly explain your reasoning.