8 The Smiths bought a car last week, ... ? haven't they? did they? .., didn't they? do they?​

8 The Smiths bought a car last week, didn't they?

In this sentence, we are looking for the correct question tag to use at the end. A question tag is a short phrase added at the end of a statement to turn it into a question and to confirm or seek agreement from the listener.

First, let's consider the statement: "The Smiths bought a car last week."

The verb used in the statement is "bought" which is in the past tense. When the statement is in the affirmative form, the question tag is usually negative. So, we can eliminate "did they?" and "do they?" as possible options.

Next, we need to determine if the subject is plural or singular. In this case, the subject is "The Smiths" which refers to more than one person, making it plural. When the subject is plural, we use "didn't" as the helping verb in the question tag.

Lastly, we need to match the pronoun used in the question tag to the subject. In this case, the subject is "The Smiths" which is a third-person plural pronoun, so the corresponding pronoun used in the question tag should be "they."

Putting it all together, we can say that the correct question tag to complete the sentence is "didn't they?" This is because "didn't" indicates the negative form and "they" matches the plural subject.

So, the correct answer to the question is "didn't they?"