Need help with correcting a sentence

Test Preparation SAT Preparation
1

SAT practice book from the college board, pg 599 question 4

During the labor dispute, barrels of potatoes were emptied across the [highway, and they thereby blocked it to all traffic.]

  1. as it is now.
  2. highway and therefore blocking it to all traffic
  3. highway, by which all traffic was therefore blocked
  4. highway, and therefore this had all traffic blocked
  5. highway, thereby blocking all traffic

The answer is the last one, no.5.
And I crossed out no.5 first when I was solving this, because there is no verb in the last sentence after the comma. Is it possible? because it is a compound sentence which contains two sentences. till highway is the first sentence and the remain part is the second sentence, but there is no proper verb in the second sentence… blocking isn’t a verb… Can anyone explain? Thanks in advance.

2

The second part is not a sentence. It’s a phrase. Therefore, it can be attached with a comma.