题目

     In a national spelling contest in America, an 11-year-old girl was asked to spell a certain word. But with her soft voice the judges were not sure if she spelled the word with the letter A or E.     16   By now, the girl knew she had misspelled the word. But instead of lying and telling the judges she had said the correct letter, she told the truth that she had said the wrong letter.     17           As the girl walked off the stage, the entire audience stood on their feet clapping to applaud her honesty. Later, dozens of newspaper reporters wrote abou已知函数，且f（x）存在最大值M和最小值N，则M、N一定满足（ ）A．M+N=8B．M-N=8C．M+N=6D．M-N=6