$5.$At an income of $$100,000,$ I spent $$90,000$ on consumer goods. When my income rose to

$$200,000,$ I spent $$160,000$ on consumer goods. My marginal propensity to consume is:

a$.0\mathrm{.}9$

b$.0\mathrm{.}8$

c$.0\mathrm{.}7$

d$.$ $$70,000$