一道语法题，帮忙看一下,谢谢~

长白山的大丈夫

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Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.
A scientists learned how to make a synthetic growth hormone, removing it painstakingly  分析A选项
B scientists had learned about making a synthetic growth hormone, they had to remove it painstakingly  分析B选项
C scientists learned how to synthesize the growth hormone, it had to be painstakingly removed  分析C选项
D learning how to make a synthetic growth hormone, scientists had to remove it painstakingly  分析D选项
E learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists  分析E选项


答案说是C,  但我觉得选C不合理。。在曼哈顿语法里有这么一句：代词在句子主语的位置上，通常会被认为指代与其平行句子的主语。C中it和scientists位置平行，但是单复数不一致，很明显应该排除？

lainemai

这句话很明显正常语序是It had to be painstakingly removed ......before scientist...... 这里before做的是一个连词，连词就是连接了两个句子，所以这个It 从语义上也不应该是指代scientist.而且其他选项错的很明显