Read the text and fill in the blank:         Last month, when William was playing football in the park with his dad, a huge dog suddenly (1)___________. The dog was very (2)____________ and wanted to play. "Why are you here by (3)___________ ?  " William asked the dog. "Let's take it home," said Dad. "I think some one has lost it. We should (4)__________ the dog for a few days. Let's try to find out where it lives."   William took a picture of the dog and added his dad's phone number. Then he (5)_________ the posters on trees in the streets near his house. The next day, a woman phoned. The dog was hers. 21. Fill in the blank (1) (1 Điểm) appearing appears appear appeared