Answer
(a) First the solid carbon dioxide changes to liquid and then to gas.
(b) First the solid carbon dioxide changes to liquid and then this liquid carbon dioxide changes to gas.
Work Step by Step
(a) We know that the solid first changes into liquid and then the liquid turns to gas when the pressure is kept constant and the temperature is increased.
(b) We know that if the temperature of the system is kept constant first below the freezing point and the pressure is decreased then the solid $CO_2$ is converted to liquid and then to gas.