Why were nonsense syllables used in the earliest studies of forgetting? so that the stimulus materials were simple enough that even children could read and remember them so that cross-cultural comparisons of memory could be investigated using speakers of different languages so that the material did not have preexisting associations in memory to reduce retroactive interference?

Nonsense syllables were used in the earliest studies of forgetting so that the material did not have preexisting associations in memory. Nonsense syllables are any of numerous letter combinations that are arbitrarily formed, used in memory or learning experiments and tests.

the factors that the economies of the muslim empires prosper were:

1- land trade routes-located between three continents  

2- sea trade routes

trade contributed to the spread of muslim empires, and with that islamic culture led to a growing feeling of internationalism resulted in the transfer of technology, science, and other cultural forms.