Widgeon Co$.$ manufactures three products: Bales, Tales, and Wales. The 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.

The contribution per machine hour for Wales is

a$.$ $$28$

b$.$ $$35$

c$.$ $$17$

$x$ d$.$ $$7$