A. the reasons craters are difficult to study B. the different shapes C. some features of large craters D. some difference in the ways small and large craters were formed 参考答案:C 答案解析:推理题。由文章第三段最后一句"The complexity of shape increases for larger craters.”可知,下文将谈到大型弹坑的特点。因此,C选项为正确答案。