Assertion $(A)$: $MgO$, $CaO$, $SrO$, and $BaO$ are insoluble in water.
Reason $(R)$: In aqueous medium, the basic strength of $MgO$, $CaO$, $SrO$, and $BaO$ increases with an increase in the atomic number of the metal.
The correct option among the following is:

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