If the subject of a sentence is separated by "and/or", should the verb be pluralized, as with "and", or agree with the rightmost subject, as with "or"? For example:

> 
> His co-workers and/or his boss was at fault.
> 
> 
> 

vs.

> 
> His co-workers and/or his boss were at fault.
> 
> 
>