In the following equation hat$(y)=30,000+4x$ with given sales $(y$ in dollars$)$ and marketing $(x$ in $$500),$ what does the equation imply?

a$.$ An increase of one unit in marketing is associated with an increase of $$32,000$ in sales.

b$.$ An increase of one unit in marketing is associated with an increase of $$2,000$ in sales.

C$.$ An increase of four units in marketing is associated with an increase of $$2,000$ in sales.

d$.$ An increase of four units in marketing is associated with an increase of $$32,000$ in sales.