Assertion : Radioactive heavy nuclei decay by a series of `alpha- and //or beta-`emission, to form a stable isotope of lead
Reaosn : Radioactivity is a physical phenomenon
A. If both assertion and reason are true and the reason is the correct explanation of the assertion
B. If both assertion and reason are true but reason is not the correct explanantion of the assertion
C. If assertion is true but reason is false
D. If the assertion and reason both are false