题目

天然皮革的主要成分是(    )A.脂肪类物质                           B.变性的蛋白质C.纤维素制品                           D.塑料制品 答案：BThey had walked eastward for a day and a night ________ they saw water and grass. [　　] A．unlessB．sinceC．afterD．before