What two things were integral to making the trans-saharan trade possible?

Trans-Saharan trade refers to merchandise traffic through the Sahara to reach Sub-Saharan Africa from the North African coast, Europe or the Levant. Although it has existed since prehistoric times, the apogee of this commercial route took place between the 8th and 16th centuries.

• Beginning in the fifth century, trade re-emerged due to the formation of the Kingdom of Ghana in the Western Sahara. The establishment of a new upper social class in the Niger-Senegal area increased the demand for luxury goods. In addition, trade was favored by the state protection offered by Ghana.

• With the establishment of the Songhai Empire from 1400, Timbuktu, on the banks of the Niger River, became the most important commercial center in the Sahel area. The disintegration of the empire after a Moroccan invasion substantially altered the trade with the central Sahara since the central government that could protect the caravan routes was missing. For this reason from the XVII century the main route of the trans-Saharan trade moves to the road between Lake Chad and Tripoli

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."