[单选题]
An ideal gas is compresse 1jhjnp 0n(bh+d isot3 tf5k vp7u.bphermally from 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 ubpf.7tp3k5 v the change of internal energy for this gas?
A. $6.0 \; J$
B. zero
C. $-6.0 \; J$
D. none of the above
