152 The AMA Handbook of Business Writing Count Nouns Count nouns are nouns that have both a singular and plural form. The plu- ral of a count noun is usually formed by adding s. Credible, Credulous Credible means believable or worthy of being believed. Credulous means inclined to believe too readily. Correct: He related the incident in a credible manner. Correct: She is too credulous for her own good. Cross-Reference A cross-reference is a link in a document to related or more detailed infor-