Assertion:An `alpha`-particle is emitted when uranium 238 decays into thorium
Reason : The decay of uranium 238 to thorium is repesented by `._92^238 U to ._90^234Th + ._2^4 He `. The helium nuclei is called an alpha particle.
A. If both assertion and reason are true and reason is the correct explanation of assertion .
B. If both assertion and reason are true but reason is not the correct explanation of assertion .
C. If assertion is true but reason is false .
D. If both assertion and reason are false