i also generally face a problem with the DO type of Q's...can anybody throw some light on this please?

think about the following three words and their variations - be, have, does

now see if any of these three words have been used in the non-underlined portion of the two questions you mention.

the tubelight should flicker on now

Dirt roads may evoke the bucolic simplicity of another century, but financially strained townships point out that dirt roads cost twice as much as maintaining paved roads.

(A) dirt roads cost twice as much as maintaining paved roads
(B) dirt roads cost twice as much to maintain as paved roads do
(C) maintaining dirt roads costs twice as much as paved roads do
(D) maintaining dirt roads costs twice as much as it does for paved roads
(E) to maintain dirt roads costs twice as much as for paved roads

I dont see variations of either do,have or be in this sentence