Between 1949 and 1989, soviet scientists exploded 470 nuclear devices in northeastern

Between 1949 and 1989, soviet scientists exploded 470 nuclear devices in northeastern Kazakhstan . After world war II this northeastern part of Kazakhstan became the nuclear weapon testing site of the Soviet Union. With the collapse of the Soviet Union regime in 1991 this testing ground was closed by Kazakhstan president. Believed clean up was done to the area to eliminate risks posed by radioactive materials.

I’m pretty sure that the answer is b.