参考答案

正确答案：C
解析：细节题。根据题干信息词a U. N. convention定位到第二个小标题末段第二句话There is a U. N. convention on trans-boundary air pollution，but that cannot cover every problem that can arise between neighbors，or between states which do not share a border，即：联合国在跨边界的空气污染上有协定，但是不包括邻国之间出现的每个问题，