You win some, then lose a few. With the unappreciative ones you just end the interview as if you never intended to offer them anything. See - nobody will force himself to do something that' s thoroughly unpleasant. And last but not least, persistance also depends on your success ratio. If you get 99 refusals for every one prospect found, you won't continue it for long - not if you're like the rest of us human beings.
Nobody can keep up something that brings little success and a lot of failures and unpleasantness.
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