Abe starts exercising regularly, and after a few months discovers he can now do twice as much of everything—in a single day Abe can now make 10 hamburgers or 8 milkshakes, rather than the 5 hamburgers and 4 milkshakes he made in the past. We now know that Abe's production possibility frontier:
A.
. has shifted right, but his opportunity costs of making milkshakes are unchanged.
B.
. has shifted right, but his opportunity costs of making milkshakes have decreased.
C.
. has not changed, but his opportunity costs of making milkshakes have increased.
D.
. has not changed, but his opportunity costs of making milkshakes have decreased.