Which one of the following is an electric conductor ?

The correct answer is C, Human body. The human body conducts electricity because its fluids contain dissolved salts that break up into ions, and these ions carry current. Blood, sweat and the fluid inside and around cells hold sodium, potassium and chloride ions, so a current can pass through the body; this is why an electric shock is dangerous and why wet skin, which lowers resistance, makes it worse. A material conducts when it has free charges, either electrons as in metals or ions as in salt solutions. Option A is wrong because a plastic sheet is a good insulator, which is why plastic and PVC are used to cover wires and switches. Option B is wrong because distilled water has almost no dissolved salts and so very few ions, making it a very poor conductor; ordinary tap water conducts only because of the impurities in it. Option D is wrong because dry wood has no free charges and is an insulator, which is why wooden handles and ladders are used near electric lines. Exam tip: ions make solutions conduct; tap water and the body conduct, distilled water hardly does.