The following data pertain to activity and utilities costs for two recent years:

Year $2$

Year $1$

Activity Level in Units

$10,000$

$6,000$

Utilities Cost

$$12,000$

$$9,000$

Using the high$-$low method, what is the cost formula for utilities?

a$.$

$$1\mathrm{.}20$ per unit.

b$.$

$$1\mathrm{.}50$ per unit.

c$.$

$$3,000$ plus $$3\mathrm{.}00$ per unit.

d$.$

$$4,500$ plus $$0\mathrm{.}75$ per unit.