6. They ... half the rubbish in the garden by the time Dad arrived. A. have cleaned up B. had cleaned up C. had been cleaning up

The correct answer is C. had been cleaning up. The sentence means that they had been cleaning up half the rubbish in the garden by the time Dad arrived. Правильный ответ: C. had been cleaning up. Предложение означает, что они убирали половину мусора в саду к тому времени, когда приехал папа.