def get_sheet_name(casc):
    """Return sheet name for provided CASC.

    Arguments:
        casc -- (string) The string name of a CASC
    Returns:
        sheet_name -- (string) the name of the corresponding sheet.

    """
    if casc.lower() == "Northwest CASC".lower():
        sheet_name = "NW"
    elif casc.lower() == "Southwest CASC".lower():
        sheet_name = "SW"
    elif casc.lower() == "Pacific Islands CASC".lower():
        sheet_name = "PI"
    else:
        return None
    return sheet_name