Each swing that susan builds requires $10 in hardware and $20 in wood. she uses the expression 10 + 20 to represent the total cost of building one swing.

which expression represents the total cost of building three swings?

a.
10 + (3 • 20)

b.
3 + (10 + 20)

c.
3 • (10 + 20)

d.
3 • 10 + 20