What genetic modification seeks to do is modifying corps in a more planned way than is possible by merely selecting healthier seeds or by producing hybrids of successful plants.
这里than后面的is为什么不省略?
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如果这是改错的话,那么错的是modifying
应该改为
What genetic modification seeks to do is modify corps in a more planned way than is possible by merely selecting healthier seeds or by producing hybrids of successful plants.
至于is要不要省略,其实属于边缘地带问题,很难有一条清晰的规则来界定。见https://www.cpsenglish.com/question/21780