请教but instead连用的问题

mogu724

Scientists who studied the famous gold field known as Serra Pelada concluded that the rich lode was not produced by the accepted methods of ore formation but that swarms of microbes over millions of years concentrated the gold from jungle soils and rivers and rocks.
A. not produced by the accepted methods of ore formation but that swarms of microbes over millions of years
B. not produced by the accepted methods of ore formation but instead swarms of microbes over millions of years that
C. not produced by the accepted methods of ore formation but swarms of microbes over millions of years that
D. produced not by the accepted methods of ore formation but by swarms of microbes that over millions of years
E. produced not by the accepted methods of ore formation but that swarms of microbes over millions of years
请问为什么说B的but instad不能连用？？？
请NN帮忙解答下 谢谢！！

mogu724

顶顶、、、、、、、求NN