Olivia sells bookshelves and small tables. It takes 1.25 hours to build a bookshelf and .9 hours to build a small table. The bookshelves have a unit contribution margin of $500 and the small tables have a unit contribution margin of $350. Olivia has been running low on time to build more products. What is her contribution margin per unit of time for the bookshelves?

A : $625

B : $400

C : $555

D : $395