Thanx 4 ur awesome explanation, but i still need J08 Q4 c 2nd varient and 4 that question y did u choose R1 and not R2??? 
okay for y i chose r1 and not r2----
this is cuz r1 is just in place to prevent too much current flowing throught the transistor and damaging it. r1 and the thermistor r acting as a potential divider; when we change the resitance of one of the 2 the voltage across the thermistor or r1 changes depending upon which components resistance we have changed. as u knw the voltage flowing across a component is governed by the resistance of the component. since we have to change the value at which the temperature at which the bulb lamps we have to change the resistance of r1. r1 is a part of the potential divider, r2 is not
ill just answer ur other q with my next post
