One Mole of an Ideal Gas Expands Isothermally and Reversibly $\Delta$

One mole of an ideal gas expands isothermally and reversibly at 0°C. The pressure on 1 mol of an ideal monatomic gas changes from 100.0 atm to 1.00 atm. Calculate $\Delta$ E.

A) 450 kJ
B) -21.0 kJ
C) -450 kJ
D) 21.0 kJ
E) none of these