Выбрать нужную форму в предложениях. Помогите пожалуйста.

1. prevents 2. being marked 3. was discovered 4. would be used 5. split 6. couldn't be removed 7. had been split _____________ 1. were sent 2. to be sliced 3. happened 4. became 5. had been fried 6. couldn't be eaten 7. were being requested.