If a 4 ω resistor, a 7 ω resistor, and a 12 ω resistor are connected in a series, which resistor has the most current in it?

series  
 current same to all resistors
 p.d  is divided to each resistors depending on their resistivity 
V = IR  the resistor which have more resistance (also resistivity) gain more p.d

parallel
 p.d  is same to all components
 current is divided to each resistor depending on their resistivity
  I = V/R  more current flows through one with less resistance (also resistivity)

here as it is series connection,current is same to all resistors irrespective of their resistance

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other