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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} (vii) B-C

$\begin{array}{1 1}(A)\;\{20\}&(B)\;\{2,6,10,14\}\\(C)\;\phi&(D)\;\{4,8,12,16,20\}\end{array}$

B={4,8,12,16,20},C={2,4,6,8,10,12,14,16}
Here 4,8,12,16 are the elements of set B are also present in set C
$\therefore B-C=\{4,8,12,16,20\}-\{2,4,6,8,10,12,14,16\}$
$\Rightarrow \{20\}$
Hence (A) is the correct answer.