Suppose, ceteris paribus, a firm's fixed costs decrease. Indicate the changes to this firm's cost curves by choosing shift upward, shift downward, or remain unchanged for each prompt that follows:
a. The firm's AFC curve would _____ (shift upward / shift downward / remain unchanged)
b. The firm's MC curve would _____ (shift upward / shift downward / remain unchanged)
c. The firm's AVC curve would _____ (shift upward / shift downward / remain unchanged)
d. The firm's ATC curve would _____ (shift upward / shift downward / remain unchanged)