The growth of the size of the gobi desert is primarily the result of

the anwser is water shortages.

just like you man, i personally think its either c or d

explanation:

but good luck with the test man

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: