In the Keyesian theory, if the marginal propensity to consume is $0\mathrm{.}90$ and government spending is decreased by $$5$ billion, then real income $($GDP$)$ will a maximum of $$ billion.

a$)$ increase; $$5$

b$)$ decrease; $$50$

c$)$ decrease; $$5$

d$)$ increase; $$50$