Due to decreased light penetration, which area of rivers and streams will have less diversity of plant life?
a. the source
b. the mouth
c. the middle portion
d. none of the above

Due to decreased light penetration, B. THE MOUTH of the rivers and streams will have less diversity of plant life.

The decreased light penetration is brought about by the murkiness of the water due to numerous deposits of sediments. Sediments that have been collected at the water travels towards the mouth of the rivers and streams causes the water to be muddy and murky making it impossible for light to penetrate. 

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.