If person A hires person B to do a job, and person B messes it up, who is more at fault?

I mean, technically person B messed up. But person A hired him. So who is more at fault?

Person A is at fault. He picked person B to do the job.
Person B is at fault. It was his job. He messed up.
Select gender and age to cast your vote:
If person A hires person B to do a job, and person B messes it up, who is more at fault?
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