The right answer is mercury inches (pressure unit of measure).
<u>Depending on the context, many pressure unit can be used:</u> Inches of mercury: This unit of measurement is used for weather measures, aviation, and fridges.
Milimeter of mercury: This unit of measurement is used to measure blood pressure (1 mmHg equals 0.00133 bar).
The inner core is solid because it is made of very dense, or heavy, materials - like iron and nickel. Even though it is very hot, these materials don't "melt" very easily, so they stay solid.