Answer:
A) Increased atmospheric radiation and long-range health problems for people living near test sites.
Explanation:
It is not B, as the United States also had a rather large stockpile of atomic weaponry, and so it is not exactly a monopoly in that sense.
It is not C, because while the United States developed the atomic bomb faster, the USSR soon outproduced nuclear weapons as compared to teh US.
It is not D, because the arms race escalated tensions between the two nations and their allies, rather than decrease it.