Understanding Evolution : Personal response question

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Understanding Evolution Personal response
question
Some head lice are now resistant to all commonly
used pesticide shampoos used to kill them. Which
is the better explanation for this observation?
a. Frequent contact with pesticide shampoos
caused mutations that allowed the lice to become
resistant so resistant lice became more and more
common. b. Some lice carried genes for resistance
to the pesticide before contact with the shampoos
and their numbers increased after lice shampoos
became common.