Consider the following multiplicative demand function where QD $=$ quantity demanded, P $=$ selling price, and Y $=$ disposable income:

$$

The coefficient of Y $($i$.$e$.,\mathrm{.}2)$ indicates that $($all other things being held constant$)$:

a$.$

for a one percent increase in disposable income, quantity demanded would increase by $\mathrm{.}2$ percent

b$.$

for a one unit increase in disposable income, quantity demanded would increase by $\mathrm{.}2$ units

c$.$

for a one percent increase in disposable income quantity demanded would increase by $\mathrm{.}2$ units

d$.$

for a one unit increase in disposable income, quantity demanded would increase by $\mathrm{.}2$ percent

e$.$

none of the above