What Soviet action in 1961 provoked US concern as far as the city of Berlin was concerned?
A) An outbreak of polio in East Berlin happened because the Soviets refused to allow the American-made vaccine into the city
B) The Soviets stopped East Berliners from crossing into West Berlin, and began constructing a wall to divide the city and contain East Berlin's population
C) A Soviet bomber struck key US targets in West Berlin
D) The Soviets placed missile launching pads on the outskirts of the portions of East Berlin that they controlled