An ideal gas expands isothermally from $10^{-3}{\text{m}}^3$ to $10^{-2}{\text{m}}^3$ at $300\text{K}$ against a constant pressure of $10^5{\text{Nm}}^{-2}$. The work done by the gas is:

1. A
   $+270\text{kJ}$
2. B
   $-900\text{J}$
3. C
   $+900\text{kJ}$
4. D
   $-900\text{kJ}$