Gamma radiations are dangerous because

Which of the following is/are correct?
i. Chain reaction takes place in a nuclear reactor and an atomic bomb.
ii. The chain reaction in a nuclear reactor is controlled.
iii. The chain reaction in a nuclear reactor is not controlled.
iv. No chain reaction takes place in an atom bomb.

${}_{Z}Y^A\to {}_{Z+1}Y^A+X\text{; then,} X$ is _____. (Choose from: e / p / n)