Which of the Following Is Not a "Clean Coal" Technology

Which of the following is not a "clean coal" technology already in use?

A) "Coal washing" to remove sulfur and other mineral impurities from the coal before it is burned.
B) "Gasification" to convert coal to a mixture of carbon monoxide and hydrogen.The resulting gas burns at a lower temperature,thus reducing the generation of nitrogen oxides.
C) "Wet scrubbing" to chemically remove SO₂ before it goes up the smokestack.This is accomplished by reacting the SO₂ with a mixture of ground limestone and water.
D) "Sequestration" to capture and store CO₂ before it is emitted into the atmosphere.