题目

 Many people agree that _____ knowledge of English is ____ must in ____ international trade today. A. /; a; the                   B. /; a; /                       C. a; a; /                      D. a; /; / 答案：C2．已知△ABC的三边长为a、b、c，且满足（a+b-c）（a-b）=0，那么△ABC是等腰三角形．