Official Solution:
When there is more rainfall than normal, the water of subtropical rivers gathers high speed and often overflows. Because Pentaflora, an aquatic species whose habitat is river water, grows best in slow-moving water, the number of Pentaflora per unit of water generally increases when there is less rainfall. By contrast, however, following a period of high rainfall, Pentaflora levels were found to be high in certain rivers which have been dammed.
Which of the following, if true, best explains the contrast described above?
A. High water speed does not cause the populations of the feeders of Pentaflora to fall.
B. The temperature of slowly moving water is more conducive for the growth of Pentaflora.
C. High Pentaflora populations increase the toxic levels of the river waters, damaging some other types of algae.
D. Damming causes subtropical rivers to flood the banks for short stints.
E. Damming enables storing and slow releasing of water along the river bed.
The correct option should explain why Pentaflora levels are high after heavy rainfall, even though high speed water is not conducive for the growth of Pentaflora. The correct option would possibly explain either
1. the possibility that the water speed is not high even though there is heavy rainfall
or
2. some other factor that helps the growth of Pentaflora even in high speed waters.
A. The population of feeders of Pentaflora does not reduce in high speed waters. If the population reduced, then this option would be an explanation why the Pentaflora population increases in high speed waters even though high speed is not conducive for the growth of Pentaflora.
B. This option explains why Pentaflora levels are high in slow moving water. It does not explain why the Pentaflora levels are high after heavy rainfall.
C. This option explains the effect of high levels of Pentaflora. It does not explain the cause of high levels of Pentaflora after heavy rainfall.
D. The passage does not state how overflowing is related to high levels of Pentaflora. This option does not explain why the Pentaflora levels are high after heavy rainfall.
E. CORRECT. This option presents a possibility that even after heavy rainfall why the water speed along the river might be low. When the rain water is stored using a dam and then slowly released along the river bed, then the water speed along the river would be low, conducive for the growth of Pentaflora.
Answer: E