true?
(a) \( P \cap Q=\phi \)
(b) \( (R \cap Q) \cup(P \cap R)=\phi \)
(c) \( P \cap Q \cap R=\phi \)
(d) all of these
19. If \( A \cap B=A \cap C \) and \( A^{\prime} \cap B=A^{\prime} \cap C \), then is it necessary that \( B=C \) ? Justify your ansie
20. In a class of 25 students, 12 have taken Mathematics, 8 have taken Mathematics but not Ris
