William returned an acoustic guitar that cost $245.75. He bought a bass guitar that cost $368.45. Which of the following statements is true? Explain your answer completely.

a. The sum 245.75 + (–368.45) = –122.70 means that Robbie still owes $122.70 for the bass guitar.
b. The sum –245.75 + 368.45 = 122.70 means that Robbie will receive a refund of $122.70.
c. The sum 245.75 + (–368.45) = –122.70 means that Robbie will receive a refund of $122.70.
d. The sum –245.75 + 368.45 = 122.70 means that Robbie still owes $122.70 for the bass guitar.