Statement $P$: The general formula of carbohydrates is $C_n(H_2O)_n$ or $C_n(H_2O)_m$.
Statement $Q$: In the structure of monosaccharides,the values of $n$ and $m$ are equal.

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