[amp_mcq option1=”a” option2=”an” option3=”the” option4=”no article” correct=”option4″]
The correct answer is: D. no article
The article “a” or “an” is used before a singular noun that refers to a general or unspecified person, place, thing, or idea. In this case, the noun “policeman” is singular and refers to a specific person who knocked on the door. Therefore, no article is needed.
The article “the” is used before a singular noun that refers to a specific person, place, thing, or idea that has already been mentioned or is known to the reader or listener. In this case, the noun “policeman” is singular, but it does not refer to a specific person who has already been mentioned or is known to the reader or listener. Therefore, the article “the” is not needed.