C.
This is also the only change by which crystals may be produced.
Conditions:
Low pressure and low temperature
Low pressure and high temperature
High pressure and low temperature
High pressure and high temperature
1 mole ------------- 22.4 L ( at STP )
?? mole ---------- 12 L
12 x 1 / 22.4 => 0.5357 moles
hope this helps!