Please choose the correct answer to the question: Just a few days ago, Beth waded into her family's pond. Now when she stepped off the bank the pond was hard and she could walk on the surface. What happened?

Answer1: a cold front had moved into the area Answer2: The pond had drained into an underground river Answer3: There was a lot of algae on the pond's surface Answer4: It had rained the night before

Answer format: answer1/answer2/answer3/answer4
Answer: answer1