What is a natural frequency?

Natural frequency is the frequency at which a system tends to oscillate in the absence of any driving or damping force. Free vibrations of any elastic body are called natural vibrations and occur at a frequency called the natural frequency. Natural vibrations are different from forced vibration which happen at frequency of applied force (forced frequency).
The letter choic would be B.

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Name at least two extinct species that share a close evolutionary relationship with humans.
Explain the similarities and differences that exist between these species and humans.

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