Two vessels A and B are connected via stopcock. The vessel A is filled with a gas at a certain pressure. The entire assembly is immersed in water and is allowed to come to thermal equilibrium with water. After opening the stopcock the gas from vessel A expands into vessel B and no change in temperature is observed in the thermometer. Which of the following statement is true?

A

$$dw \neq 0$$

B

$$dq \neq 0$$

C

$$dU \neq 0$$

D

The pressure in the vessel B before opening the stopcock is zero.