In the earth’s layers, why is the outer core in the state of a liquid and the inner core in the state of a solid?

a. the temperature of the core will not allow either part to be solid.

b. the outer core is hotter that an inner core which makes the material liquid.

c. the composition of the outer layer is made up of material that has a lower melting point

d. the pressure applied to the inner core causes it to be solid even though it is hotter than the outer core.

(( this is science))