Which of the following most logically completes the argument below?
Within the earth's core, which is iron, pressure increases with depth. Because the temperature at which iron melts increases with pressure, the inner core is solid and the outer core is molten. Physicists can determine the melting temperature of iron at any given pressure and the pressure for any given depth in the earth. Therefore, the actual temperature at the boundary of the earth's outer and inner cores-the melting temperature of iron there-can be determined, since ______.
【选项】nowhere in the earth's core can the temperature be measured directly