helium 密度低，同一频率声音波长会变大，同一根共振管本身长度不变，对应谐振频率必须变大了，所以外面谐振的声音的频率就要增加。看完答案后的理解，但觉得答案给的解释并不好
B.
The change in timbre results in a higher perceived frequency of the voice.
Explanation：The pitch of a perceived sound is dependent on its frequency. In the case of the human voice, there are many overlapping waves with various frequencies and higher harmonics of those frequencies. Nonetheless, the basic fact remains that pitch depends on perceived frequency. Thus, when helium atoms are inhaled, the density of the air is being changed, which alters the timbre (an overall quality of voice that results from overlapping waves) to emphasize the higher-pitched harmonics. We perceive that as a high-pitched, "squeaky" voice. However, note that even without knowing the definition of timbre, this question can be solved by eliminating the other options.
C.
The frequency decreases due to the change in air density from helium atoms.
