20. A circuit consists of a resistor R=10 Ω and an inductor L=1 H in series connected to a direct circuit source of 5 V. The time constant of the circuit is   (20)  

19. A Carnot engine operates between two heat reservoirs at temperatures of 400 K and 800 K. The maximum efficiency of the engine is   (19)   

18. Light in an optical fiber undergoes total internal reflection. If the index of refraction of the fiber is 1.5 and that of the surrounding air is 1.0, the critical angle for the light traveling in the fiber is   (18)  

80 Which of the following is mentioned as an illegal activity in a foreign country? (A) Feeding pigeons in Venice (B) Riding a bicycle on sidewalks in Scotland (C) Eating on the street in Singapore (D) Taking pictures of US government buildings

79 What does ‘them’ in the last sentence refer to? (A) destinations (B) countries (C) laws (D) passports

78 In the passage, what does the word ‘nuances’ most closely mean? (A) Legal differences (B) Subtle differences (C) Cultural variations (D) Major contrasts