Dynamic RAM (DRAM) stores each bit of data in a separate capacitor. Due to leakage, the stored charge tends to dissipate over time and needs to be refreshed periodically. Consider the following statements:
P: DRAM requires refreshing because it uses capacitors to store bits. 

Q: SRAM does not require refreshing because it uses flip-flops instead of capacitors.
R: DRAM is faster than SRAM because it needs less frequent access.
S: DRAM is more suitable for main memory than SRAM due to its density.