def complement_base(base, material ='DNA'):
    """Returns the Watson-Crick complement of a base."""
    if base == 'A' or base == 'a':
        if material == 'DNA':
            return 'T'
        elif material == 'RNA':
            return 'U'
    elif base == 'T' or base == 't' or base == 'U' or base == 'u':
        return 'A'
    elif base == 'G' or base == 'g':
        return 'C'
    else:
        return 'G'