En que división del periodo preclásico se desarrollaron técnicas de cultivo e intercambio comercial.

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.

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:

google it and it will pop up