A、A whole week. B、A whole night. C、Three days. D、Twenty-four hours. D

In November 1965, New York was blacked out by an electricity failure. The authorities promised that it would not happen again. Pessimists believed that it would occur again within five years at the latest. In July 1977, there was a repeat performance which produced varying degrees of chaos throughout the city of eight million people. In 1965, the failure occurred in the cool autumn and at a time of comparative prosperity. In 1977, the disaster was much more serious because it came when unemployment was high and the city was suffering from one of its worst heat waves.
    In 1965, there was little crime during the darkness, and fewer than a hundred people were arrested. In 1977, hundreds of stores were broken into. Mobs smashed shop windows and helped themselves to rob jewelry, clothes or television sets. Many people were arrested but far more disappeared into the darkness of the night. The number of policemen available was quite inadequate and they wisely refrained from using their guns against mobs which far outnumbered them and included armed men.
    Hospitals had to treat hundreds of people cut by glass from shop windows. Banks and most businesses remained closed the next day. The blackout started at 9:30 p. m. , when lightning hit and knocked out vital cables. Many stores were thus caught by surprise.
    The vast majority of New Yorkers, however, were not involved in robbing. They helped strangers, distributed candles and batteries, and tried to survive in a nightmare world without traffic lights, refrigerator, elevators, water and electrical power. For twenty-four hours, New York realized how helpless it was without electricity.
19. In what way was the blackout of 1977 different from that of 1965?
20. Why did many mobs manage to escape?
21. What caused the blackout in July 1977?
22. How long did the 1977 New York electricity failure last?