Because serfs did everything for them, and without serfs they would have to do them themselves
Migration:-
Migration is to leave the country you live in, your own country and go live in another country you chose forever.
Answer:Belgrade fell again in October 1915, as the Bulgarians made their way into Niš and Kosovo. All looked lost for the Serbs, so the decision was made to retreat through the mountains of Montenegro and Albania in the hope of making it to Greece, the nearest stronghold of Serbia's allies.
Explanation:
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To trade with the west indies, new england and africa
