Read the passage carefully, then choose an option (A, B, C or D) that best answers each question.

An estimated 18,000 people die every day worldwide as a result of air pollution. The great majority of the world's population breathe air that does not meet World Health Organization guidelines. Air pollution has become so bad that it's said we now have a “fifth season”: this time of year, when lethal smogs envelop some of the most populated parts of the world. Delhi's atrocious smogs, which caused an international cricket match to be halted on Sunday, follow similar ones last year.

But 65 years on from the toxic Great Smog of London that descended on 5 December 1952, and led to ground-breaking anti-pollution laws being passed, the air above the UK still hasn't cleared. In London alone more than one person an hour dies prematurely from a range of conditions such as congestive heart failure, asthma and emphysema as a result of exposure to particulate matter and nitrogen dioxide. The mayor of London, Sadiq Khan, has called for a new Clean Air Act that would enshrine a right to clean air.

Smog Day marks the anniversary of the Great London Smog, and the middle of the international smog season. It grew out of an initiative to share the experiences of people living with air pollution in London and New Delhi, whose air quality is among the worst in the world. In spite of many differences between life in the two capital cities, there are parallels in the experiences of people who work on the streets, runners who exercise along them, taxi drivers, parents and children and the doctors who care for those with breathing difficulties.

Progress on air pollution is already being made in many places around the world. The recent Lancet Commission on pollution and health points out that air-quality improvements not only save lives, but have other benefits.

36. What is the main idea of the passage?