Cho hai tập hợp: \(X = \left\{ {7;8;9} \right\};Y = \left\{ {1;3;7;4} \right\}\) .

A. \({\left\{ {1;2;3;4;8;9;7} \right\}}\)

B. \({\left\{ {1;3;4;7;8;9} \right\}}\)

C. \({\left\{ {1;3} \right\}}\)

D. \({\left\{ {2;8;9;12} \right\}}\)