Assertion $(A)$: $Be$ and $Mg$ do not impart any colour to the flame.
Reason $(R)$: The electrons in them are too strongly bound to get excited by the energy of the flame.
The correct option among the following is:

• A
  $(A)$ and $(R)$ are true, $(R)$ is the correct explanation of $(A)$
• B
  $(A)$ and $(R)$ are true, but $(R)$ is not the correct explanation of $(A)$
• C
  $(A)$ is correct but $(R)$ is incorrect
• D
  $(A)$ is incorrect but $(R)$ is correct