Two identical thermally isolated containers are separated by a valve. Initially, there is an ideal gas in container 1, and there is a vacuum in container 2. Some time after the valve is opened, the gas in the two containers comes to an equilibrium state. Which of the following statements about the gas during this process is true?
(a) The molar mass of the gas decreases.
(b) The work produced by the gas is zero.
(c) The temperature of the gas drops.
(d) The work produced by the gas is positive and is equal to the absolute value of the change of the internal energy of the gas.