Assertion $(A)$: $Mo$ has the ground state electronic configuration $4d^5 5s^1$.
Reason $(R)$: $Mo$ has the highest exchange energy among the second row transition elements.
The correct option among the following is:

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