In an experiment, scientists control the

variables
controls
biologicals
hypotheses

the president has the power to appoint federal judges, ambassadors, and other "principal officers” of the united states, subject to senate confirmation of such appointments.

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.