A gas expands under constant pressure P from volume V1 toV2. The work done by the gas is

(1) $P\left({V}_{2}-{V}_{1}\right)$

(2) $P\left({V}_{1}-{V}_{2}\right)$

(3) $P\left({V}_{1}^{\gamma }-{V}_{2}^{\gamma }\right)$

(4) $P\frac{{V}_{1}{V}_{2}}{{V}_{2}-{V}_{1}}$