Read the following paragraph and select the correct answer.

shaking off from my spirit what must have been a dream, i scanned more narrowly the real aspect of the building. its principal feature seemed to be that of an excessive antiquity. the discoloration of ages had been great. minute fungi overspread the whole exterior, hanging in a fine tangled web-work from the eaves. yet all this was apart from any extraordinary dilapidation. no portion of the masonry had fallen; and there appeared to be a wild inconsistency between its still perfect adaptation of parts, and the crumbling condition of the individual stones. in this there was much that reminded me of the specious totality of old wood-work which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air. beyond this indication of extensive decay, however, the fabric gave little token of instability. perhaps the eye of a scrutinising observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn. (from "the fall of the house of usher," by edgar allan poe)
which sentence best illustrates the main idea of this paragraph?
the discoloration had been great.
its principal feature seemed to be that of an excessive antiquity.
yet all this was apart from any extraordinary dilapidation.
perhaps the eye of a scrutinising observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.