Why is this English sentence wrong? Could you help me find out the reason ?

Hello. I'm a foreigner learning English. (I live in Asia.) Now I'm in a difficult situation. My questions might be simple for you, but not for me. This is important for me. I should be very grateful to you if you might help me.. - As long as you answer me, I hope you are a native. But I will welcome... show more Hello. I'm a foreigner learning English. (I live in Asia.)
Now I'm in a difficult situation. My questions might be simple for you, but not for me. This is important for me.
I should be very grateful to you if you might help me.. - As long as you answer me, I hope you are a native. But I will welcome whoever you are if you know English well enough.-

The decision to cut back on production was not sudden, ( ) was rather the result of several months of research.
A: which B: but (I omitted the rest of examples.)
Do you think which of the examples(A or B) is suitable(right) for the blank?
I chose A('which') but it's wrong. I looked up the word, 'rather', in a dictionary, (I found out this structure; 'but rather')

My question ; The problem is that I can't understand the reason 'which' is wrong'.

I know the use of ', which' (in relative clauses).
- The decision to cut back on production was not sudden, which was rather the result of several months of research.-
This sentence doesn't seem to be wrong to me.
Is the reason why 'but' has to be used... because 'rather' exists in the sentence?
If 'rather' left out, is it possible that 'which' is the answer? Is the following sentence right?
- The decision to cut back on production was not sudden, which was the result of several months of research.-
If not, does 'but' have to be used in the sentence if 'rather' dropped?

I'm not sure..exactly. The more I think about it, The more confused I get.
I need your help. Really, I'll appreciate your help.
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