Answer:
The correct option is B) Currents prevent accumulation of most organic matter.
Step-by-step explanation:
In fast- flowing river, organic matter will not get accumulated. The organic matter will be in constant motion due to the currents. Hence, life would be very difficult in fast- flowing rivers. The organisms living in fast- flowing rivers will not receive enough food as there will be minimal organic matter. Also,due to the currents, the organisms will have difficulty maintaining themselves. Hence, very few species live in fast-flowing rivers.