Which excerpt from nadine gordimer's “once upon a time” best prepares the reader for the symbolic meaning of the dragon's teeth as it relates to the development of the plot?
…the low-cost option of pieces of broken glass embedded in cement along the top of walls, …
… (twelve-inch pikes finned like zigzags of lightening and painted white).
… it consisted of a continuous coil of stiff and shining metal serrated into jagged blades, so that there would be no way … through its tunnel without getting entangled in its fangs
… sunlight flashed and slashed, off the serrations, the cornice of razor thorns encircled the home, shining.

   The excerpt from Nadine Gordimer's "Once Upon a Time" that best prepares the reader for the symbolic meaning of the dragon's teeth, is:

   "Placed the length of walls, it consisted of a continuous coil of stiff and shining metal serrated into jagged blades, so that there would be no way of climbing over it and no way through its tunnel without getting entangled in its fangs. There would be no way out, only a struggle getting bloodier and bloodier, deeper and sharper hooking and tearing of flesh. The wife shuddered to look at it. You're right, said the husband, anyone would think twice... And they took heed of the advice on a small board fixed to the wall: Consult DRAGON'S TEETH The People For Total Security."

  That's how we can summarize "Once Upon a Time": After waking at night from strange sounds, fearing she doesn't have enough security, a frightened writer tells herself a tale about a modern family, that for the potential dangers around them have their perfect life destroyed for the sake of security; having to appeal to a  "ultimate security device", the Dragon Teeth. After several security measures they install this type of spiked wire on the walls outside their house, believing no one could possibly escalate the walls afraid of being fatally injured; what comes as unexpected is that the little boy of the family, not aware of the danger, tried to escalate the walls on a play:

"One evening, the mother read the little boy to sleep with a fairy story from the book the wise old witch had given him at Christmas. Next day he pretended to be the Prince who braves the terrible thicket of thorns to enter the palace and kiss the Sleeping Beauty back to life: he dragged a ladder to the wall, the shining coiled tunnel was just wide enough for his little body to creep in, and with the first fixing of its razor-teeth in his knees and hands and head he screamed and struggled deeper into its tangle"

That is how the Dragon's teeth and the excerpt relate to the development of the plot.