第24题: [单项选择]氧元素有()种同位素。 A. 1种 B. 2种 C. 3种 D. 4种 参考答案:C
第25题: [单项选择]While they were away on vocation, they allowed their mail to (be accumulated) at the post office. A. be decreased B. be delivered C. be piled up D. be returned 参考答案:C 答案解析:be piled up“堆积”。be decreased“减少”;be delivered“邮递,传递”;be returned“返回到…”。由上可知,be accumulated意思最接近的词组为b