If a Production Possibility Curve Representing a Trade-Off Between a Grade

If a production possibility curve representing a trade-off between a grade in English and a grade in math has a negative slope, we know that:

A) there is a direct relationship between grades in English and grades in math.
B) there is no relationship between grades in English and grades in math.
C) there is an inverse relationship between grades in English and grades in math.
D) one can get better grades in English only if one gets better grades in math.