Consider the following statements : (a) Boolean expressions and logic networks correspond to labelled acyclic digraphs. (b) Optimal Boolean expressions may not correspond to simplest networks. (c) Choosing essential blocks first in a Karnaugh map and then greedily choosing the largest remaining blocks to cover may not give an optimal expression. Which of these statement(s) is/ are correct? (A) (a) only (B) (b) only (C) (a) and (b) (D) (a), (b) and (c)