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.