Which calendar first divided dates into two categories bc and ad

Gregorian christian calendar

Explanation:

I hope this helps

1. darius the great     persia  

2. piankhi       nubia

3. ezana       aksum

4.     nebuchadnezzar     babylonia

5. shalmaneser iii     assyria

the causes of the dust bowls were severe droughts,   lands became dry and hard, temperature was higher than usual, hot and dry conditions killed crops, high winds. because of those factors, the winds kicked up dry soil and created clouds of dust.