$A$: Microbes have been used for the welfare of humanity.
$R$: The fields of biotechnology and genetic engineering have developed.

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