Answer:
a. Residual service activities
Step-by-step explanation:
The primary sector would the extraction for nature activities like crops, mining and finish among other of similar characteristics
Then, the secondary sector would the conversion to finished good of the raw materials. All the manufacturing of textiles, engines, ships, tools, and other products
While the tertiart secotr would be service activities like telegraph, the incipient energy, trasnport, and other when there is no conversion to a finished good from a raw material.