Assertion : The ionic radii of the alkaline earth metals are smaller than those of alkali metals in the same period.
Reason : Alkali metals have higher nuclear charge than that of the alkaline earth metals.

• A
  $A$. $(A)$ is true,$(R)$ is true and $(R)$ is the correct explanation for $(A)$
• B
  $B$. $(A)$ is true,$(R)$ is true but $(R)$ is not the correct explanation for $(A)$
• C
  $C$. $(A)$ is true but $(R)$ is false
• D
  $D$. $(A)$ is false but $(R)$ is true