Scientists found a patch of grass growing in Manyikeni that is the same species as the grass that grows near Great Zimbabwe--and only there. What does this discovery suggest to historians?

that Arab traders from Egypt stabled their camels in Manyikeni
that the farmers in Manyikeni learned techniques from farms in Great Zimbabwe
that Manyikeni was an outpost for trade between Great Zimbabwe and the port at Kilwa
that the wind blew from Great Zimbabwe to Manyikeni, carrying grass seed

Question

Scientists found a patch of grass growing in Manyikeni that is the same species as the grass that grows near Great Zimbabwe--and only there. What does this discovery suggest to historians?
 	
	that Arab traders from Egypt stabled their camels in Manyikeni
	that the farmers in Manyikeni learned techniques from farms in Great Zimbabwe
	that Manyikeni was an outpost for trade between Great Zimbabwe and the port at Kilwa
	that the wind blew from Great Zimbabwe to Manyikeni, carrying grass seed