If the opportunity cost of Joan increasing her grade was constant regardless of how many hours she worked. Which shapes would be more likely curve

If the opportunity cost of Joan increasing her grade was constant regardless of how many hours she worked. Which shapes would be more likely curve shape or constant shape and why ?

What would be happening if Joan's combination of her grades and hours of work per week were below the curve?

How could Joan push her combination of her grades and hours of work per week past the current curve?

please provide references