Final answer:
The common nouns in the sentence 'Within three hours, nine inches of rain had fallen in Clay County.' are hours, inches, and rain, referring to units of time, distance, and precipitation.
Step-by-step explanation:
The question asks to identify the common nouns in the sentence: "Within three hours, nine inches of rain had fallen in Clay County." Common nouns are nouns that name general items rather than specific ones, which are known as proper nouns. In this sentence, the common nouns are hours, inches, and rain. They refer to units of time, distance, and precipitation respectively, without specifying a particular hour, inch, or rain event that would make them proper nouns.