问一个GMAT句子改错
The lack of complete historical records from the mid-to-late 1800's has made it difficult in tracing some Black inventions to their originators.
正确答案是把in tracing改成to trace
给出的原因解释是 作为形式宾语的代词it,不能向后指代介词短语in tracing 而可以向后指代不定时短语to trace
尤其是形式宾语的向后指代问题