Widgeon Co$.$ manufactures three products: Bales, Tales, and Wales. Their selling prices are $$55,$$$78,$ and $$32,$ respectively. The variable costs for each product are $$20,$ $$50,$ and $$15,$ respectively. Each product must go through the same processing in a machine that is limited to $2,000$ hours per month. Bales take $5$ hours to process; Tales, $7$ hours; and Wales, $1$ hour.

What is the contribution margin per machine hour for Wales?

a$.$ $$17$

b$.$ $$28$

c$.$ $$35$

d$.57$