When you pull a cotton sweater off your skin, electrons are transferred from the ​ (a)   (cotton, skin) to the ​ (b)   (cotton, skin). As a result, your body acquires a ​ (c)   (+ , -) charge and the cotton sweater acquires a ​ (d)   (+ , -) charge.

(a)   are materials that hinder electrons to flow freely from particle to particle.

When you get shocked by touching a doorknob, that's an example of (a)   .