So am stuck on a question:

The PPF between goods X & Y will be a downward - sloping

A.) Straight line if increasing opportunity costs exist.

B.) Straight line if decreasing opportunity costs exist.

C.) Curve that is bowed outward if increasing opportunity costs exist.

D.) Curve that is bowed outward if constant opportunity costs exist.

I know the law of opportunity costs is when the more you produce the costly it becomes (the short version) but am not quite sure how it would effect the shape of the slope. Please help.

Question

So am stuck on a question: 

The PPF between goods X & Y will be a downward - sloping 

A.) Straight line if increasing opportunity costs exist.

B.) Straight line if decreasing  opportunity costs exist.

C.) Curve that is bowed outward if increasing opportunity costs exist.

D.) Curve that is bowed outward if constant opportunity costs exist. 

I know the law of opportunity costs is when the more you produce the costly it becomes (the short version) but am not quite sure how it would effect the shape of the slope. Please help.