The melting and boiling points of four substances are: P $200\ \text{K}$ and $300\ \text{K}$; Q $240\ \text{K}$ and $400\ \text{K}$; R $260\ \text{K}$ and $380\ \text{K}$; S $300\ \text{K}$ and $600\ \text{K}$. Which one is a liquid both at $250\ \text{K}$ and at $350\ \text{K}$?