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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def replace_body_vars(self, body):
 """Given a multiline string that is the body of the job script, replace the placeholders for environment variables with backend-specific realizations, and return the modified body. See the `job_vars` attribute for the mappings that are performed. """

        for key, val in self.job_vars.items():
            body = body.replace(key, val)
        return body