What are your body's first line of defense? How does your skin work to protect you?

Skin is a barrier that serves as one of the body's first lines of defense against harmful microbes. Specialized immune cells within skin tissue help to fight invading organisms. Past work by an NIH research team showed that commensals can help the body's immune cells fight disease-causing microbes.
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Skin blood bones hope that helped

Option a): level has been with us for a long time as early as at least 3rd century BC.,

b) is a fairly recent development around 1950s

c) electricity has been discovered in 17th century, so before b)

d.) theory of evolution dates back to the middle of 19th century, so before b)

therefore b) is the best answer.