[单选题]
The work done on an ideal gas system in an isothermal process is kr,wfw:3l td d 3a+sd7-400 J. Whuteu6 22wmssjgkraa1:hz1 16at is t mkuh 66s: 2a1z e1g1w2atrsujhe change in internal energy?
A. zero
B. $-400 \; J$
C. $400 \; J$
D. none of the above
