The presence of helium gas indicates the radioactive sample is most likely decaying by α-decay, or alpha decay. α-decay is the type of radioactive decay in which an atomic nucleus emits α particles. α particles are Helium nuclei. So the correct answer would be α-decay.
Answer:
The right solution is "24.39 per sec".
Explanation:
According to the question,
⇒ 

The time will be:
⇒ 



hence,
⇒ 

'Universe' means 'Everything'. That is, all matter, all space, all time.