def get_only_x(window_c, window_x):
    """
    Of the two time windows, return the period where only factor x is observed.

    Parameters:
        window_c: the window where c is observed.
        window_x: the window where x is observed.
    
    Returns:
        A list of two: start and end time of the window where only x is observed.
    """
    r, s = window_c
    p, q = window_x

    # if c happens before x, get the latter part starting when c ends
    if r < p:
        return((s, q))
    # when x starts first, get the first part until c starts
    elif p < r:
        return((p, r))
    # when both windows are the same, return None
    else:
        return(None)