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

• A
  $I$ is false but $II$ is true.
• B
  $I$ is true but $II$ is false.
• C
  $I$ and $II$ are true.
• D
  $I$ and $II$ are false.