<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def set_zones(token, action, relay=None, time=None):
 """ Controls the zone relays to turn sprinklers on and off. :param token: The users API token. :type token: string :param action: The action to perform. Available actions are: run, runall, stop, stopall, suspend, and suspendall. :type action: string :param relay: The zone to take action on. If no zone is specified then the action will be on all zones. :type relay: int or None :param time: The number of seconds to run or unix epoch time to suspend. :type time: int or None :returns: The response from the controller. If there was an error returns None. :rtype: string or None """

    # Actions must be one from this list.
    action_list = [
        'run',        # Run a zone for an amount of time.
        'runall',     # Run all zones for an amount of time.
        'stop',       # Stop a zone.
        'stopall',    # stop all zones.
        'suspend',    # Suspend a zone for an amount of time.
        'suspendall'  # Suspend all zones.
    ]

    # Was a valid action specified?
    if action not in action_list:
        return None

    # Set the relay id if we are operating on a single relay.
    if action in ['runall', 'stopall', 'suspendall']:
        if relay is not None:
            return None
        else:
            relay_cmd = ''
    else:
        relay_cmd = '&relay_id={}'.format(relay)

    # Add a time argument if the action requires it.
    if action in ['run', 'runall', 'suspend', 'suspendall']:
        if time is None:
            return None
        else:
            custom_cmd = '&custom={}'.format(time)
            period_cmd = '&period_id=999'
    else:
        custom_cmd = ''
        period_cmd = ''

    # If action is on a single relay then make sure a relay is specified.
    if action in ['stop', 'run', 'suspend'] and relay is None:
        return None

    get_response = requests.get('https://app.hydrawise.com/api/v1/'
                                'setzone.php?'
                                '&api_key={}'
                                '&action={}{}{}{}'
                                .format(token,
                                        action,
                                        relay_cmd,
                                        period_cmd,
                                        custom_cmd),
                                timeout=REQUESTS_TIMEOUT)

    if get_response.status_code == 200 and \
       'error_msg' not in get_response.json():
        return get_response.json()

    return None