[单选题]
An ideal gas is compressed isotd 2l-g9ion(ppfcj d- ;hermally froa9mehq 4t;l2p m 30 L to 20 L. During this process, 6.0 J of energy is expended by the external mechanism that compressed the gas. What is the ch9ephat2mq ;4l ange of internal energy for this gas?
A. $6.0 \; J$
B. zero
C. $-6.0 \; J$
D. none of the above
