Which layer of soil has little or no humus

O Horizon - The top layer of soil made up mostly of leaf litter and humus. 

The answer is d., a mountain.

The answer is c, i, iii, and iv. this is because farming and mining deal with harvesting raw materials, construction work falls under manufacturing, and piloting a plane is a service. ii is wrong because making bread is using  raw materials to produce goods, which would fall under the secondary sector, not primary.