Pusan produces flash drives for computers, which it sells for $$20$ each. Each flash drive costs $$10$ of variable costs to make. During April, $1,000$ drives were sold. Fixed costs for March were $$1,000$ for the month. How much is the contribution margin ratio?

a$.$

$75\%$

b$.$

$20\%$

c$.$

$35\%$

d$.$

$50\%$