Deduce the missing monomial.

a) [[$ 4a \cdot $]] [[$ = 20a^2$]]
b) [[$ 3b^2 \cdot $]] [[$ = 18ab^3$]]
c) [[$ 5bc \cdot $]] [[$ = -15ab^4c^5$]]
d) [[$ -2b^6d^4 \cdot $]] [[$ = 18b^7d^6$]]