关于but作连词前面是否应该用逗号，有疑问

lisa001

In her presentation, the head of the Better Business Bureau emphasized that companies should think of the cost of conventions and other similar gatherings as not an expense, but as an investment in networking that will pay dividends.

A. as not an expense, but as 


B. as not expense but 


C. not an expense, rather 


D. not as an expense, but as 


E. not in terms of expense, but 


本题我选对了，OG 19的题目。就是有个疑问，but在这里作连词连接一个有修饰的noun前面加逗号，是不是因为修饰成分比较长？它通常连接一个名词，我好像记得不用加逗号的？求指导～～