Many transition metals form interstitial compounds. The characteristics of these interstitial compounds are:
$I$. They have high melting points,higher than those of pure metals.
$II$. They are very hard.
$III$. They retain metallic conductivity.
$IV$. They are chemically more reactive than the pure metals.

• A
  $I, II, III$
• B
  $I, III$
• C
  $II, IV$
• D
  $IV$ only