Refer to the Graph

Refer to the graph.
When you play piano, why might you have to hit the keys harder with your left hand (lower notes) than your right hand (higher notes) to make them sound the same volume?

A) Low-frequency sounds are more condensed, making them sound louder.
B) High-frequency sounds are less condensed, making them sound softer.
C) Higher-frequency tones require higher dB levels to sound the same volume as lower-frequency tones.
D) Lower-frequency tones require higher dB levels to sound the same volume as higher-frequency tones.
E) The audibility threshold curve is lower for low-frequency sounds and higher for high-frequency sounds.