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# If A={3,6,9,12,15,18,21},B={4,8,12,16,20},C={2,4,6,8,10,12,14,16},D={5,10,15,20} (iii) A-D

$\begin{array}{1 1}(A)\;\{3,6,9,12,15,18,21\}&(B)\;\{3,6,9,12,18,21\}\\(C)\;\{5,10,15,20\}&(D)\;\phi\end{array}$

A={3,6,9,12,15,18,21}
D={5,10,15,20}
Here 15 is the only element of set A is common to set D.
$\therefore A-D=\{3,6,9,12,15,18,21\}-\{5,10,15,20\}$
$\Rightarrow \{3,6,9,12,18,21\}$
Hence (B) is the correct answer.