Use the following information to answer the question:

C $=$ $$600$ billion $+0\mathrm{.}9($Yd$)$; I $=$ $$850$ billion

Equilibrium is at income $($real GDP$)$ equal to $\_\_\_\_\_.$

a$)$

$$11,000$ billion

b$)$

$ $15,000$ billion

c$)$

$ $19,500$ billion

d$)$

$ $14,500$ billion