"Metals are usually not found as nitrates in their ores."
Out of the following two $(a)$ and $(b)$ reasons,which is/are true for the above observation?
$(a)$ Metal nitrates are highly unstable.
$(b)$ Metal nitrates are highly soluble in water.

• A
  $a$ and $b$ are false.
• B
  $a$ is false but $b$ is true.
• C
  $a$ is true but $b$ is false.
• D
  $a$ and $b$ are true.