ABSTRACT
A · B · C + A · B · C + A · B · C + A · B · C (8) 11. A clean room has two entrances, each having
two doors, as shown in Fig. A3.1. A warning bell must sound if both doors A and B or doors C and D are open at the same time. Write down the Boolean expression depicting this occurrence, and devise a logic network to operate the bell using NAND-gates only. (8)
