Concept:“Dirt” is an uncountable noun, so it cannot be used with the article “a.”
Explanation:In this sentence, a noun is needed to complete the blank.
“Dirty” is an adjective, so option A is wrong.
“Dirtiness” is an abstract noun that does not fit the intended meaning, so option C is wrong.
“A dirt” is incorrect because uncountable nouns cannot take “a.”
Instead, use the quantifier “some” with the uncountable noun “dirt.”
Therefore, the best choice is “some dirt.”
Answer:D. some dirt