Consider the following radioactive decays

I. ${}_{92}\mathrm{U}{\stackrel{-\mathrm{\alpha }}{⟶}}_{90}\mathrm{Th}$ and

II. ${}_{90}\mathrm{Th}\stackrel{-\mathrm{\alpha }}{⟶}{}_{88}\mathrm{Ra}$

In which case group of parent and daughter elements remains unchanged.

• Correct answer
  A

  In $\left(\mathrm{I}\right)$
• B

  In $\left(\mathrm{II}\right)$
• C

  Both in $\left(\mathrm{I}\right)$ and $\left(\mathrm{II}\right)$
• D

  None of the above

Hence, the correct answer is option: 'A'.