已知全集U＝{1,2,3,4,5,6,7,8}，M＝{1,3,5,7}，N＝{5,6,7}，则 (M∪N)＝(　　)

A：{5,7}

B：{2,4}

C：{2,4,8}

D：{1,3,5,6,7}