Concept:A starter provides nitrogen to activate composting and speed up decomposition of organic matter.
Explanation:The diagram shows a compost-making method where waste materials are layered and left to decompose.
Urine contains a high amount of nitrogen, which is an essential nutrient for compost microorganisms.
These microorganisms require nitrogen to grow, multiply, and break down plant and animal waste efficiently.
Therefore, urine acts as a compost activator or starter.
Ash and straw are poor nitrogen sources, so they are not suitable starters.
Blood also contains nitrogen but is less commonly used and practical than urine in this context.
Answer:D. Urine
