A decrease in the price of a good will usually lead to:

a$)$ An increase in the demand for the good due to its increased affordability $($income effect$)$

b$)$ A decrease in the demand for substitutes of the good as they become relatively less attractive

c$)$ Both $($a$)$ and $($b$)$

d$)$ Neither $($a$)$ nor $($b$)$