8. The children — to plant flowers since… — to plant a lot of primroses.

Длительное действие и его результат: have been planting и have planted.

The children have been planting flowers since … — they have planted a lot of primroses.

Перевод: Дети сажают цветы с … — они посадили много примул.

Ответ: The children have been planting flowers since … — they have planted a lot of primroses.