What brought an end to the system of serf labor?
a. lords found cheaper workers for their lands.
b. peasants replaced serfs in the fields.
c. workers were in short supply.
d. lords decided serf labor was unfair

"b. Peasants replaced serfs in the fields." would be the best option from the list, but it should be noted that serfs still existed in on the property for other tasks, although these tasks were usually preferable to those on the field. 

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