$4.$ Replace the AND operations by ORs and vice versa by applying De Morgan's laws to each of these expressions: $[$Hint: refer to textbook page$\mathrm{.}373$ Example $7\mathrm{.}12]$ a$.$ F$=$AB$+($C$+$A$)$ D b$.$ F$=$A$($B$+$C$)+$D c$.$ F$=$ABC$+$A$($B$+$C$)$