Please help! this is timed

(E) When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal-background noise.

Explanation:

E is true and cast most doubt because it facilitates an experimental approach towards a given assumption. The mantees were exposed to the recorded sounds of the boats they were actually incapable of perceiving the change in sound and this clearly weakens the given argument that mantees are unable to hear the low pitched sounds when the boat get slow .