An ideal gas undergoes an isothermal process. Which of the following statements about the change in internal energy (U) for the gas and heat (Q) is true during this process?
a. U is zero, and Q is zero.
b. U is zero, and Q is not zero.
c. U is not zero, and Q is zero.
d. U is not zero, and Q is not zero, but they are not equal.
e. U is not zero, and Q is not zero, and they are equal.