Answer:
Two Half-lives
Explanation:
Let number of Parent nuclei Initially present be X,
Then, finally 
Parent nuclei Will remain with 
daughter nuclei.
In one half- life , parent nuclei becomes half of initial.
So, starting with X parent nuclei,
After one half-life, it will degrade to 
.
After another half life , Parent nuclei will become half of
Which is equal to .
So, Parent nuclei have to go through Two half-lives.
I would say B.
Explanation: Peer review improves the quality of research papers.
Yes, S-blocks are metals except for helium and hydrogen. The S-block metals are shiny, silvery and relatively soft, although they can easily lose electrons. Helium and hydrogen have valence electrons but also lack some similar properties as the other metals.
