When my wife, who is Spanish, spent her first winter in London a few years ago, she used to ask me time and again：“Where’s the fog?”Almost all foreigners 1 to find the city full of yellow – grey fog for most of the years．Dickens, who was 2 responsible for painting this 3 in people’s minds, certainly wasn’t joking in those days．In the nineteenth century people used to say 4 someone attempted to kill himself by 5 into the Thames, he would be choked by the fog and poisoned by the terrible 6 of the river before he had time to drown himself．In fact, the situation has 7 only in recent years．When I was a boy in London thirty years ago, I was often unable to see 8 of the road when I left home on winter mornings．
The key steps that have turned London into one of the 9 cities in the world were 10 at the end of the 1950s．But Londoners still find it 11 that fog hardly returns．Actually the 12 took place as a result of two main improvements．Factories were forced to fix equipment for air cleaning 13 close down, and local people were not allowed to 14 coal unless it was smoke – free．But the 15 incredible miracle in London occurred 16 1964 when the Thames Water Authority began to pump vast 17 of dissolved oxygen into the river． 18 , varieties of fish that had gradually disappeared from the Thames since 1800 have 19 , and some are 20 caught by fishermen outside the House of Parliament(议会)．