In 1958 trash collection programs were established, and backyard incinerators were banned in the hope of reducing smog. however, it was estimated that massive amounts of gasoline were evaporating from storage tanks, fuel pump nozzles, and delivery trucks. this was also considered a serious contributor to smog, and over the next 20 years, several new rules and several new devices were put in place to alleviate it. there was still a great deal of smog, now identified as photochemical smog, despite the changes. the major remaining problem was

The major remaining problem was the large distances driven daily by solitary commuters

Explanation:

Photochemical smog occurs when nitrogen oxides and volatile organic compounds mix together in the presence of sunlight which acts as a catalyst and form ozone at lower levels.

Even though measures such as ban of incinerators and regulation of evaporation of gasoline were instated  photochemical smog was still present because people drive for a long time to get to work.

As far as photochemical smog is concerned it the combustion of other fossil fuels, such as gasoline by motor vehicles and industry is seen as its main contributor. Combustion releases primary pollutants: volatile organic compounds and nitrogen oxides. These pollutants are the main cause of photochemical combustion.

b) to encourage free trade

hygrometers and hair thermometers (often found as part of hygrometers/psychrometers) can be used to measure humidity. barometers are used to measure atmospheric pressure, while anemometers are used to measure wind speed. so the 2nd and 4th choices should be the correct answers.