20. The following fgure shows a conductive sphere with a spherical cavity in the center. The total charge on the conductor is zero, but a point charge Q is placed at the center of the cavity. How does the electric potential (V) as a function of the distance from the center (r) looks like?(A)(B) (C)(D)(E)

複選題22. Consider a damped harmonic oscillator, which of the following is true?(A) If it is underdamped, the resonant frequency increases with larger damping； (B) If it is underdamped, the amplitude decays faster with larger damping； (C)If it is underdamped, and is driven by an external sinusoidal force, the amplitude of the oscillation is maximum if the frequency of the force is the same as the resonance frequency； (D) In order to damp the kinetic energy and the elastic energy as fast as possible, one should design the oscillator to be at critical damping condition； (E) If it is overdamped, the system does not oscillate and returns to the equilibrium position slowly.

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