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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 add_var_mb(table: PrettyTable, vardict: Dict[str, str], varname: str) -> None: """ Adds a row to ``table`` for ``varname``, in megabytes. """

    valstr = vardict.get(varname, None)
    table.add_row([varname, val_mb(valstr), UNITS_MB])