An ideal gas of mass $$m$$ is contained in a rigid tank of volume $$V$$ at a pressure $$P.$$ During a reversible process its pressure reduces to $${P_1}.$$ Following statements are made regarding the process.
$$(P)$$ Heat is transferred from the gas.
$$(Q)$$ Work done by the gas is zero.
$$(R)$$ Entropy of the gas remains constant.
$$(S)$$ Entropy of the gas decreases.

Among the above statements, the correct ones are