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

　　E. but their references were falsified as well

这题选C，D错哪里了？but不是可以省略吗？