【请教】not only..but also怎么用？ PREP 1-147 V.S 大全987-354

关于not only...but also 的用法还是不清楚，疑问如下：

大全987  354.  In 1978 a national study found that not only had many contractors licensed by a self-policing private guild failed to pass qualifying exams, they in addition falsified their references.

(A) they in addition falsified their references

(B) they had their references falsified in addition

(C) but they had also falsified their references

(D) they had also falsified their references（C）

         (E) but their references were falsified as well

此题答案C，因为要求前后对象的形式对称，这里时态也一致，不知道这一要求是不是NOT ONLY BUT ALSO结构所要求的

但是C中but also中间有they had 这出现在PREP中，如下：

prep  147. (32726-!-item-!-188;#058&006292)
By pressing a tiny amount of nitrogen between two diamonds to a pressure of 25 million pounds per
square inch, scientists not only were able to transform the gas into a solid, but they also created a
semiconductor similar to silicon.
(A) not only were able to transform the gas into a solid, but they also created
(B) not only were able to transform the gas into a solid but also creating
(C) were able not only to transform the gas into a solid but to create
(D) were able not only to transform the gas into a solid but also creating
(E) were not only able to transform the gas into a solid, but they were also able to create

句子结构：By pressing… scientists were able not only to … but to… 平行结构。
考点：平行对称
平行对称-not only… but also…，注意连接对象的形式对称（also 可以省略）。
(A) not only…but also连接对象不对称，but also之间的they多余。
(B) not only…but also连接对象不对称，were able to transform与现在分词creating。
(C) 正确，not only to transform… but to create…
(D) not only…but also连接对象不对称，to transform与现在分词creating不对称。
(E) not only…but also连接对象不对称，but also之间的they were多余。

我的疑问是如果大全里but also中插入they had是正确的用法，那怎么排除PREP此题的E选项？？

pls