Two coupled gas-phase isomerisations occur at constant volume and temperature: $\mathrm{A}\rightleftharpoons \mathrm{B}$ with $K_1=4.0$, and $\mathrm{B}\rightleftharpoons \mathrm{C}$ with $K_2=0.50$. The mixture has $[\mathrm{A}]=0.50$, $[\mathrm{B}]=1.0$ and $[\mathrm{C}]=0.80\mathrm{mol}{\mathrm{L}}^{-1}$. Which statement describes the initial net changes?

1. A$[\mathrm{B}]$ decreases, while $[\mathrm{A}]$ and $[\mathrm{C}]$ both increase
2. B$[\mathrm{B}]$ increases, while $[\mathrm{A}]$ and $[\mathrm{C}]$ both decrease
3. C$[\mathrm{A}]$ increases and $[\mathrm{C}]$ decreases
4. D$[\mathrm{B}]$ increases and $[\mathrm{C}]$ also increases