DNS MX Question : Multiple choice Question

I was wondering what would happen to incoming mail in this scenario...

webhost.com MX 10 supermail.com
webhost.com MX 20 backupmail.com


Choice A) Mail goes to supermail.com ONLY
If supermail.com is down, only then it goes to backupmail.com

Choice B) Mail goes to supermail.com AND backupmail.com


I believe ChoiceA is correct. But I need to implement ChoiceB. I was thinking of keeping both on same MX-Priority but not sure it will work perfectly.

Input from DNS experts needed on this !

 

 

 

 

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