请教not only ...but(also)与 not only... but ...as well的优劣

1.        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

若把D改成 but they hd also falsified their references as well,则在C和D中应选哪个

要么but also---

要么but---as well

是这么说吧？怎么有but also---as well的说法的？