A farmer was put in prison . One day, he got a letter from his wife. "I am worried about our farm," she wrote. "It's time to plant potatoes, but I can't do all the digging  by myself." The farmer thought over and then had an idea. He wrote to his wife, "Don't dig the fields. This is where my gold  is. Don't plant potatoes until I come home." A few days later, the farmer got another letter from his wife. It said, "Two days ago, about ten prison guards  came to our fields. It looked as if they were looking for something. They have dug our field." The farmer wrote to his wife at once. "Now you can plant our potatoes," he wrote. Why did the farmer ask his wife to plant potatoes at once? Because   _  .
their fields had been dug the gold was found out the prison guards asked him to do so the prison guards were digging the fields