$A$: Atoms with the same mass number but different atomic numbers are called isobars.
$R$: In isobars,the sum of protons and neutrons is always different.

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

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