Answer: B
Explanation:
The reactivity of alkaline metal ( example group one ) increases down the group because of the increase in atomic mass from the top to the bottom of the group. As the mass increases, the force between the nucleus and the valence electron gets weaker and the atom will easily give up the electron as a result of this which eventually increases its reactivity.
The reactivity of halogens decreases as atomic mass increases. Therefore, option B is the correct answer.
