def c(units):
    """Speed of light

    Parameters
    ----------
        units : str
            Units for c. Supported units

            =====  =====================  ===========
            Unit   Description            Value
            =====  =====================  ===========
            m/s    Meter per second       299792458
            cm/s   Centimeter per second  299792458e2
            =====  =====================  ===========

    Returns
    -------
        c : float
            Speed of light in appropriate units
    Raises
    ------
        KeyError
            If units is not supported.
    """
    c_dict = {
        'm/s': 299792458.,
        'cm/s': 299792458.e2,
    }
    try:
        return c_dict[units]
    except KeyError:
        err_msg = ('Invalid unit: {}. Use help(pmutt.constants.c) for '
                   'accepted units.'.format(units))
        raise KeyError(err_msg)