def wanmen_download_by_course_topic_part(json_api_content, tIndex, pIndex, output_dir='.', merge=True, info_only=False, **kwargs):
    """int, int, int->None
    
    Download ONE PART of the course."""

    html = json_api_content

    title = _wanmen_get_title_by_json_topic_part(html, 
                                                  tIndex, 
                                                  pIndex)

    bokeccID = _wanmen_get_boke_id_by_json_topic_part(html,
                                                      tIndex, 
                                                     pIndex)

    bokecc_download_by_id(vid = bokeccID, title = title, output_dir=output_dir, merge=merge, info_only=info_only, **kwargs)
int, int, int->None
    
    Download ONE PART of the course.