How did the crusades contribute to the decline of feudalism?

a. they increased trade with the middle east, and at the same time saw more people resettle in prosperous cities, or leave the countryside.
b. they allowed the ottoman turks to invade europe and destroy the feudal system.
c. they ended up costing so much money that lords could no longer afford to pay for their manors and had to sell them.
d. they destroyed the power of kings, meaning the feudal system broke down.