it could be better to

[joham]

---When shall we hold the meeting?
---It _____ be better to put it off until next week.
A. must B. could C. can D. will

The given answer is B. But I would think D works as well. And I searched BNC, and got the numbers of results for:
It could be better to... 2
It must be better to... 3
It will be better to... 4
It may be better to... 30
It might be better to... 50

I don't understand.
When used for showing that sth is/was possible, 'may', 'might', 'could', and 'will' can all be used.
If the speaker is making a suggestion, then 'might' and 'could' can be used. But in this sentence pattern, which is better?

Might I ask native English teachers to help me to solve my puzzle?
Thank you very much.

 

[stuartnz]

I'm not a teacher, but I would prefer "could" over "will". The first option conveys uncertainty or polite deference, an acknowledgment of the other party's interests. To say "it will be better to" is making an expression or statement as if of fact. When I reschedule appointments, I almost say "it could be better to postpone for a week" or "it might be better to postpone for a week". I would seldom if ever say "it will be better to to postpone for a week."