why didn't the USSR respond with force to the East German and Polish revolts, as they had back in the 50s and 60s?

Eric Hobsbawm, the Marxist British historian, wrote a book called The Short Twentieth Century. The 20th Century had been shorter than other centuries because it had begun in 1914 with the outbreak of the First World War and terminated of course early in November 1989 with the fall of the Berlin Wall. The problem however, and of course we historians we like problems, is that everybody knew what we had left behind with the fall of the wall, but nobody knew what we were heading towards. As Douglas Hurd, the British Foreign Secretary at the time, put it, “this was a system [the Cold War], this was a system under which we had lived quite happily for 40 years.” Or as Adam Michnik, again my Polish solidarity intellectual, put it “The worst thing about communism is what comes afterwards.” While our populations were in jubilation in front of the television screens or on the streets of Berlin, governments were, it has to be said, seriously worried about the implications of this unforeseen, uncontrolled and uncontrollable collapse of the Warsaw Pact and the communist system. Tom Wolf, the American author, at the time had a bestseller called the Bonfire of the Vanities and a British MP that I knew at the time famously rephrased that as the ‘bonfire of the certainties.’ All of the reference points with which we’d lived for half a century and which had organized our diplomacy, our military strategy, our ideology, were like as many props that were suddenly pulled from us.

d is the answer. leo should include the source.