Solid $\mathrm{CaCO}{}_3$ is in equilibrium with $\mathrm{CaO}$ and $\mathrm{CO}{}_2$ in a cylinder with a movable piston at constant temperature, with excess solid present. The volume is doubled and equilibrium is re-established. Compared with the original state, the pressure of $\mathrm{CO}{}_2$ and the amount of $\mathrm{CO}{}_2$ are

1. Apressure halved, amount unchanged
2. Bpressure halved, amount doubled
3. Cpressure unchanged, amount unchanged
4. Dpressure unchanged, amount doubled