25. Which of the following properties is wrong for a semiconductor? (A) Obeying Ohm's law. (B) Can be doped to increase conductivity. (C) Can make light-emitting diodes. (D) Can make transistors.

4. A point-like bug of mass m lies on the rim of a uniform disk of mass M that can rotate freely about its center. Initially the bug and disk rotate together with an angular velocity ω₀. Then the bug walks half way to the center of the disk. What is ratio of the new kinetic energy of the system to its initial kinetic energy? (A) (4m+2M)/(m+2M) (B) (m+2M)/(4m+2M) (C) (2m+4M)/(2m+M) (D) (2m+M)/(2m+4M)

25. Which of the following properties is wrong for a semiconductor?
(A) Obeying Ohm's law.
(B) Can be doped to increase conductivity.
(C) Can make light-emitting diodes.
(D) Can make transistors.

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