Biological warfare, also known as germ warfare, is the use of biological toxins or infectious agents such as bacteria, viruses, insects, and fungi with the intent to kill, harm or incapacitate humans, animals or plants as an act of war.
Serfs/Peasants would live on the protected land of the lord's manor in exchange for food and goods. The serfs/peasants basically fueled the manor and created self sufficiency, to the point where some lords even forbade Serfs/Peasants to leave their estate.
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You’re right the answer is B