Final answer:
To improve their search, researchers should utilize their library catalog and databases and consider checking additional search engines or a metasearch engine.
Step-by-step explanation:
When a web search engine does not index a significant portion of the web and does not provide enough relevant and credible sources, a researcher can take additional steps to enhance their search. One option is to search their library catalog and databases, which may contain scholarly resources that are not easily accessible on the web. Another step is to check additional search engines or use a metasearch engine, which can aggregate results from multiple search engines and provide a broader range of sources. Therefore, the researcher should take both option b and c, searching their library catalog and databases and checking additional search engines or a metasearch engine.
Learn more about strategies to enhance web searches