问题
选择题
| There were some chairs left over _____ everyone had sat down. |
A. when B. until C. that D. where |
答案
答案：A
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| There were some chairs left over _____ everyone had sat down. |
A. when B. until C. that D. where |
答案：A
连在苯环上显中性，化学式为C7H7NO2的有机物中其分子结构中有一个苯环，两个侧链（间位），符合下列酸碱性条件的结构简式为：
，所以可用乙二醇、对苯二甲酸跟上述既显酸性又显碱性的物质发生反应，生成高分子化合物[(C17H13NO5)n]。该化合物为高强度纤维材料，其结构简式为_______。