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