(8) Our gathering today is being broadcast throughout Western Europe and North America. (9) I understand that it is being seen and heard as well in the East. (10) To those listening throughout Eastern Europe, a special word: Although I cannot be with you, I address my remarks to you just as surely as to those standing here before me. (11) For I join you, as I join your fellow countrymen in the West, in this firm, this unalterable belief: Es gibt nur ein Berlin. [There is only one Berlin.]

(12) Behind me stands a wall that encircles the free sectors of this city, part of a vast system of barriers that divides the entire continent of Europe.

Which of the following sentences, if placed before sentence 12, would provide the most effective introduction to the topic of the paragraph?

a
And yet, this one and only Berlin is not free.

b
For this reason, Berlin is a city with tight borders.

c
In fact, there is one thing that has helped to keep the city unique.

d
Many cities have erected walls, and Berlin is no different.

e
Take, for example, the Berlin Wall.