You probably filled the space in with a noun which can be made into a plural (a countable noun) and which refers to a period of time, e.g.

a

day week month year

ago

The reason that the noun needs to be countable is because of the indefinite article 'a' modifying it. Uncountable nouns cannot normally be modified by the indefinite article. So, you can say 'a chair' and 'three chairs' but you can't say *'a furniture' or *'three furnitures'. The words in the above list therefore obey a grammatical constraint associated with the use of the indefinite article.

The reason that the noun needs to be a time word is because of the need for semantic consistency with the postmodifier 'ago'. Hence the above list contains choices which obey constraints at two linguistic levels at the same time, grammatical and semantic.

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