def METHOD_NAME(cpus: Optional[str] = None,
                              memory: Optional[str] = None,
                              disk_tier: Optional[str] = None) -> Optional[str]:
    del disk_tier  # unused
    if cpus is None:
        cpus = f'{oci_conf.DEFAULT_NUM_VCPUS}+'
    if memory is None:
        memory_gb_or_ratio = f'{oci_conf.DEFAULT_MEMORY_CPU_RATIO}x'
    else:
        memory_gb_or_ratio = memory
    instance_type_prefix = tuple(
        f'{family}' for family in oci_conf.DEFAULT_INSTANCE_FAMILY)
    df = _get_df()
    df = df[df['InstanceType'].notna()]
    df = df[df['InstanceType'].str.startswith(instance_type_prefix)]
    logger.debug(f'# get_default_instance_type: {df}')
    return common.get_instance_type_for_cpus_mem_impl(df, cpus,
                                                      memory_gb_or_ratio)