Assertion $A$: Alkali metals impart characteristic colors to the flame in the flame test.
Reason $R$: They have very low ionization enthalpy.

• A
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• B
  Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$.
• C
  $A$ is true but $R$ is false.
• D
  Both $A$ and $R$ are false.