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Would you help me with another question?

From an asian Toefl grammar book locally published.

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3 The union ____ two dissimilar organisms live together is called "symbiosis."
A. by which
B. which
C. whose
D. what

Answer : A
My Opinion : I agree to the answer.
As for the reason why (c) cannot be the answer,
Is it because union has the meaning of action and state?

Thanks.

Whose (the possessive of 'who') is usually confined to people.

I'd quite like "The union in which. . . " or "by means of which" but they're not options.

No, that's not the reason here. 'Whose' is possessive but the union does not possess the two dissimilar organisms any more than the nail possesses the two pieces of wood, or the glue the wallpaper and the wall.