第21题: [单项选择]Visitors to Yellowstone Park ______ consider their impact on the natural environment, leaving behind litter and other damage. A. nearly B. seldom C. ever D. yet 参考答案:B 答案解析:[词汇] 副词seldom(很少,不常) [错误答案] (C)ever(永远……)和(D)yet(还未……)如果要表达否定的意思,就要在否定句中使用。因此两个都是错误答案。