Why did early humans use slash-and-burn agriculture a. to get rid of pest b. to keep undergrowth from drying out the earth c. to keep the soil turtle d. ro get rid of old crops

Im pretty sure its A and C but if i had to choose one i would go with A

ITS A

the answer to your question is:

strong-mayor system

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.