Suppose Andrea is taking just two courses and is at a point on her PPF of grades for those two courses. Now this PPF shifts inward and Andrea moves to a point on the new PPF. Which of the following would be impossible after her PPF has shifted inward compared to before the PPF shifted? Group of answer choices a.one of her grades to rise and the other grade to fall b.both of her grades to fall c.one of her grades to fall while the other grade stays constant d.both of her grades to rise