A research project requires searching for the effect of water fluoridation on caries rate. Which is the most appropriate search phrase for this topic?

Correct answer: A. "Water fluoridation" AND caries

• A. CORRECT, and it wins on two counts. The QUOTATION MARKS bind 'water fluoridation' into an exact phrase, so the database returns only records where those two words appear together in that order — excluding papers that happen to mention water and fluoridation separately in unrelated contexts. The AND operator then requires caries to also be present, narrowing the results to the intersection of both concepts, which is exactly the research question.
• B. This is the closest wrong answer and would retrieve usable results, but without quotation marks 'water' and 'fluoridation' are treated as separate terms that may appear anywhere in a record. It is less precise than the phrase-bound version.
• C. OR is a broadening operator — it returns records containing EITHER concept. This produces the largest and least relevant result set of the four, the opposite of what a focused review needs.
• D. Mixing AND with OR without parentheses creates ambiguous precedence, and the OR before caries broadens the search enormously — it would return every paper about caries regardless of any fluoridation content.

Note: Boolean operators: AND narrows (both terms required) · OR broadens (either term) · NOT excludes. Quotation marks force an exact phrase; truncation with an asterisk (fluorid*) catches fluoride, fluoridation and fluoridated at once; parentheses control precedence when combining operators. A well-built PICO question translates directly into this syntax.