Prime Implicants
EACH of the coverings is a PRIME IMPLICANT.
Minimum SOP will have some or all of these prime implicants. The included prime implicants must cover all of the ONEs.
F(A,B,C,D) = BC’ + A’B’D (minimum # of PIs) = BC’ + A’B’D + A’C’D (valid, but not minimum) ? A’B’D + A’C’D (both PI’s, but all ‘1’s not included!)