What is the body's first line of defense against infection?
a. the heart
b. the lungs
c. the skin
d. the spleen

~Hello there! ^_^

Your question: What is the body's first line of defense against infection..?

Your The body's first line of defense against infection is the skin.

The answer is option C.

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The body's first line of defense against infection is the skin.

Infection, most of the time, comes from the outside. 
So your first line of defense would be on the very outside.

The heart is on the inside. Wrong.
The lungs are on the inside. wrong.
The skin is on the outside. Correct.
The spleen is on the inside. Wrong.

The only answer is Answer Choice C.