补出have还是补出do的区别？

Gmat苏醒者

1There is a widespread belief in the United States and Western Europe that young people have a smaller commitment to work and a career than their parents and grandparents and that the source of the change lies in the collapse of the "work ethic."

A a smaller commitment to work and a career than their parents and grandparents

B less of a commitment to work and a career than their parents and grandparents

C a smaller commitment to work and a career than that of their parents and grandparents

D less of a commitment to work and a career than their parents and grandparents had

E a lessening of the commitment to work and a career that their parents and grandparents had

2The guiding principles of the tax plan released by the Treasury Department could have even a greater significance for the economy than the particulars of the plan.

• A
  
  even a greater significance for the economy than
• B
  
  a significance that is even greater for the economy than
• C
  
  even greater significance for the economy than have
• D
  
  even greater significance for the economy than do
• E
  
  a significance even greater for the economy than have
• 比较这两个例子，按照第二题的选择，主句是have，从句要补出的是do，而不是have本身，而do在从句中是可省可不省的，那为什么第一题要把had补出来呢，我大概能够理解歧义方面的解释，但是不理解为什么补出的是had，而不是didi，如果第一题补出had是对的，那为什么第二题不是选have呢
• 
  

1998JIN

因为正确严谨的语法应该是补出do，所以第二题只能选do的那个
而第一题出的并不严谨，但是没办法，根据句意优先原则，即使你明知道补出do这里犯了点语法错误你也得选句意正确的d，而不能选择明显存在歧义的b

AMBER513

第一句的时态不一样，对于young people来说是have,但是对于their parents and grandparents来说是had;第二句的时态是一致的（The guiding principles of the tax plan released by the Treasury Department could have even a greater significance for the economy than the particulars of the plan could have.）这里的could have用do 代替了，避免显得wordy