Step-by-step explanation:
Without laws that protect children from too much physical work, the future of these children is dark. This sentence already supports the idea presented in sentence 7. To add further support for this idea, sentence D would be the best option to replace sentence 8:
Children who spend long hours helping in agriculture learn a lot from real and practical education. This sentence provides additional information to support the idea that children working in agriculture is a common occurrence, but it also highlights the negative consequences of such work and the need for protective laws.