Read the passage below and choose one correct answer for each question.

I often hear or read about ‘natural disasters’ - the eruption of Mount St. Helen, a volcano in the state of Washington. Hurricane  Andrew in Florida, the floods in the America Midwest, terrible earthquakes all over the world, huge fires, and so on. But I'll never forget my first personal experience with the strangeness of nature - the London Killer Fog of 1952. It began on Thursday, December 4,  when a high-pressure system of warm air covered southern England.

With the freezing-cold air below, heavy fog formed. Pollution from factories, cars and coal stoves mixed with the fog. The humidity was terribly high, there was no breeze at all. Traffic such as cars, trains, boats stopped. People couldn’t see, and some walked onto the railroad tracks or into the river. It was hard to breathe, and many people got sick. Finally, on Tuesday, December 9, the wind came and the fog went away. But after that, even more people got sick. Many of them died.

What didn’t happen during the time of the ‘London Killer Fog’?

A. Pollution

B. Heavy rain

C. Humidity

D. Heavy fog