$\begin {array} {ll}
\text{(i) {P,R,I,N,C,A,L}} & \text{(a) \{x : x is a positive integer and is a divisor of 18\}}\\
\text {(ii) {0} } & \text {(b) \{x: x is an integer and } x^2-9=0 \text{\}} \\
\text {(iii) {1, 2, 3, 6, 9, 18}} & \text{(c) \{x : x is an integer and x + 1= 1\} }\\
\text {(iv) {1,2,3,6,9,18}}& \text{(d) \{x : x is a letter of the word PRINCIPAL\}}
\end {array}$