Annic Jump Cannon, ________discovered so many stars that she was called “the census taker of the sky”.
句意：著名的天文学家Annic Jump Cannon发现了很多恒星，她被称为天空的人口调查员。a leading astronomer是Annic Jump Cannon的同位语，who作为关系代词，代替主语。故正确答案为A。