题目

________ moment he saw Zhao Benshan’s funny performances, he burst into _______ laughter.   A. A; /        B. The; /       C. A; the       D. The; a 答案：B已知a、b、c、d∈R，且a+b=c+d=1,ac+bd＞1.求证：a、b、c、d中至少有一个是负数.