Solve the Problem $\$ 9000$ Last Year from a Part-Time Job $8 \%$

Multiple Choice

Solve the problem.
-Walt made an extra $\$ 9000$ last year from a part-time job. He invested part of the money at $8 \%$ and the rest at $7 \%$ . He made a total of $\$ 680$ in interest. How much did he invest at each rate?

A) $\$ 5000$ at $7 \% ; \$ 4000$ at $8 \%$
B) $\$ 4000$ at $7 \% ; \$ 5000$ at $8 \%$
C) $\$ 4500$ at $7 \% ; \$ 5000$ at $8 \%$
D) $\$ 4000$ at $7 \%$ ; $\$ 7000$ at $8 \%$