Which layer is made up of iron and nickel? *

Crust
Mantle
Inner Core
Lithosphere

The core is made of two layers: the outer core, which borders the mantle, and the inner core. The boundary separating these regions is called the Bullen discontinuity. The outer core, about 2,200 kilometers (1,367 miles) thick, is mostly composed of liquid iron and nickel.

Explanation:

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

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