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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} (viii) B-D

$\begin{array}{1 1}(A)\;\{5,10,15\}&(B)\;\{4,8,12,16\}\\(C)\;\{4,8,12,16,20\}&(D)\;\phi\end{array}$

B={4,8,12,16,20}
D={5,10,15,20}
Here 20 is the only element is common to set B & D
$\therefore B-D=\{4,8,12,16,20\}-\{5,10,15,20\}$
$\Rightarrow \{4,8,12,16\}$
Hence (B) is the correct answer.