19. Which of following statements is not true for conductors, insulators, and semiconductors? (A) For conductors, the highest occupied band is only partly filled with electrons. (B) For insulators, the Fermi energy is lower than the top the valence band. (C) For semiconductors, electrons can be excited to the conduction band by thermal and/or electrical excitation. (D) For p-type semiconductors, the majority charge carriers are holes.

9. In a model gasoline engine, the air-gasoline mixture can be treated approximately with a thermodynamic cycle known as the Otto cycle. In the first process (1→2), the gas undergoes an isentropic compression (the engine's compression stroke), during which the temperature rises from 300 K to 600 K. The next process (2→3) is an isochoric pressure increase, and the gas temperature reaches 1500 K as the gasoline burns. Next (3→4), the gas undergoes isentropic expansion (the engine's power stroke) during which its temperature drops to 800 K. Finally (4→1), the gas experiences an isochoric pressure decrease during which its temperature drops back to 300 K. Please estimate the efficiency of this model engine. (A) 10%~20% (B) 20%~30% (C) 30%~40% (D) 40%~50%.

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