The monitoring results of a city's environmental protection department showed that the air quality of the city was at an excellent level as a whole in the first half of this year. The number of severely polluted days was less than 15 days, and the number of polluted days was 4 days less than the same period last year. It can be seen that this year's air quality is better than last year. which is the most questionable conclusion? ['The city shut down several heavily polluting companies last year.', 'The city has launched afforestation activities this year.', 'The severely polluted period of the city is generally in the second half of the year.', "The city's environmental protection department updated its air detection equipment this year."]