the usage of "contribution"

[Takashiro]

I was told that "contribution" is either countable or uncountable. But later a question came up.

The Red Cross made ( ) great contribution to the salvation of many victims in the Wenchuan Earthquake.
A. a B. / C. the D. an

I am completely puzzled `cause I think Both Option A and B are correct. Nonetheless, I chose A and it's really the answer. I wonder the adjective "great" is the reason why only Option A is correct.

[Alan]

Hi.

I would choose 'a' because it suggests 'one contribution'. Using 'the' would not be appropriate because that would suggest it is a contribution that has been referred to before or the only one.

Alan
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