10. Which sentence in the paragraph contains an error in verb usage?

(1) Liz and Kyle were nearly done with the realistic medieval sand castle they were building for the annual contest. (2) Suddenly the weather turned stormy. (3) If the judge have not studied their entry before a big wave came ashore, it would have been ruined before it could be judged. (4) Instead, they now held a 2nd prize trophy, even though their castle was gone.
10 points
sentence 1
sentence 2
sentence 3
sentence 4

sentence 3: If the judge have not studied their entry before a big wave came ashore, it would have been ruined before it could be judged.