4 Tenants in SFH - whose name should the utility bill be under?

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Hi Everyone! I'm stuck and would love any advice you might have...

I am currently renting out my 4 BR SFH. I have 4 separate tenants under 4 separate rental agreements. My clause does state that utilities "are not included in the rental sum...and are the responsibility of all the tenants in the household". My question is, how do I determine which tenant takes over the bills?

The utilities have been under my name but with the recent disputes and headaches, I would like them to take over. 

Thank you in advance!!

Cindy