Pick out the statement which is not relevant in the discussion of colloids.

• A
  Sodium aluminium silicate is used in the softening of hard water.
• B
  Potash alum is used in shaving rounds and as antiseptic in medicine.
• C
  Artificial rain is caused by throwing electrified sand on the clouds from an aeroplane.
• D
  Deltas are formed at a place where the river pours its water into the sea.