What is the cause of resistance?

What can be the difference between the specific resistance for a thin and thick wire made of the same material?

How much current will an electric bulb of resistance $1100 \ \Omega$ draw from a $220 \ V$ source? If a heater of resistance $100 \ \Omega$ is connected to the same source instead of the bulb,calculate the current drawn by the heater.

The resistivity of a material does not change if:

$A$ potential difference $V$ is applied across a conductor of length $L$ and diameter $D$. How is the resistance $R$ of the conductor affected,when in turn $(i)$ $V$ is halved,$(ii)$ $L$ is halved,and $(iii)$ $D$ is doubled? Justify your answer in each case.

What will happen to the resistance of the circuit,if the current through it is doubled?