[单选题]
An ideal gas is compressed isotqh65ibxi :dggtzr *c i/s;;c9hermally fr ,svuhr2m+2pjj h; 2bpom 30 L to 20 L. During this process, 6.0 J of ener22pvr sh, j jmu2h+b;pgy is expended by the external mechanism that compressed the gas. What is the change of internal energy for this gas?
A. $6.0 \; J$
B. zero
C. $-6.0 \; J$
D. none of the above