| APA | Smart Define. (1970). opportunity knocks at the door only once thesaurus. Retrieved September 30, 2026, from https://smartdefine.org/opportunity_knocks_at_the_door_only_once/thesaurus/broader |
| Chicago | Smart Define. 1970. "opportunity knocks at the door only once thesaurus" https://smartdefine.org/opportunity_knocks_at_the_door_only_once/thesaurus/broader (accessed September 30, 2026). |
| Harvard | Smart Define 1970, opportunity knocks at the door only once thesaurus, Smart Define, viewed 30 September, 2026, <https://smartdefine.org/opportunity_knocks_at_the_door_only_once/thesaurus/broader>. |
| MLA | Smart Define. "opportunity knocks at the door only once thesaurus" 1 January 1970. Web. 30 September 2026. <https://smartdefine.org/opportunity_knocks_at_the_door_only_once/thesaurus/broader> |