题目

   Mark Twain was a great writer. He was from the USA. He was born in 1835. He was also a famous speaker. He was famous for his sense of humour. Many people liked to listen to him talk because he liked to tell some interesting stories to make people laugh all the time.    One day Mark Twain was going to a small town because of his writing. Before he was going to leave, one of his friends said to him that there were always a lot of mosquitoes in the town and told him that he’d better not go there. Mark Twain waved (摇动) his hand and said, “It doesn’t matter. The mosquitoes 已知是二元一次方程组的解，则2m-n的算术平方根为（   ） A．±2              B．             C．2                D．4