What do the mountains around Mexico City trap and why is this a bad thing?

Try not to breathe in Mexico City.” That’s the advice of one newspaper columnist as the capital endures one of its worst pollution emergencies in years.

This sprawling megalopolis has been blanketed in smoky haze since last weekend, the result of dozens of forest fires and hot, windless weather at the end of the dry season.

Explanation:

i believe your answer is d)

answer: the time zones used to be whatever the local authority wanted them to be. one town might have 6am whenever the sun came up and another town had 6am 10 minutes later.  

the trains had a hard time when their orders said to be at a certain point at 6am.  

6am, but who's 6am.  

the railroads got together and set the time zones more or less along the longitude lines across the country.

explanation: