20. Which one of the following statement of superconductor is wrong? (A) The resistivity goes to zero. (B) When temperature increases higher than its critical temperature, the superconductivity will disappear. (C) The electric field inside a superconductor must be zero. (D) The magnetic field inside a superconductor must be zero. (E) The conductivity goes to infinity.

4. Consider a collision between two objects. Which one of the following statements is wrong? (A) If the collision is elastic, the total kinetic energy is conserved after the collision. (B) If the collision is elastic, the total momentum is conserved after the collision. (C) If the collision is inelastic, the internal energy of these two objects may increase. (D) If the collision is inelastic, the total momentum is not conserved after the collision. (E) If the collision is inelastic, the total kinetic energy is not conserved after the collision.

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