1. Polly has just brought Coca-Cola.2. Ron hasn't bought his Halloween costume yet. 3. Have they bought flowers yet?4. Have they made sandwiches yet?5. They have just bought hot dog.6. Andrew have already brought a DVD player.7. They have already decorated the room.Во втором предложении вопросительного знака не стояло? Если мы употребляем слово yet, то предложение либо является вопросом, либо это отрицательное предложение.