How did uranium become present in earth's upper crust? choose one: a. it was delivered by comets that crashed into earth's surface. b. it developed naturally in the upper crust. c. it rose upward as it was carried within magma. d. it does not exist in the upper crust. uranium only exists in the mantle.

Option A. It was delivered by comets that crashed into Earth's surface.

Explanation:        

Uranium (U) is a chemical element with atomic number 92.        

For many years, a large number of scientists have been studying the abundance and origin of the isotopes of uranium in Earth. According to some theories, the Earth's uranium was produced in one or more supernovae (an explosive brightening of a star), in wich, the main process consists in the rapid capture of neutrons by seed nuclei at great rates. Another theory proposes that uranium is created during the merger of two neutron stars (neutron stars are very dense), because, when such dense bodies come closer together the gravitational force cause them to merge, producing huge amounts of hevy metals like uranium.                  

Many analyses have been made of the uranium in rocks of the Earth. These measurements shows that the abundance of uranium is bigger in the crust and upper mantle of the Earth.    

So, knowing that Earth's uranium was produced through one of these processes, the best answer is option A, the uranium was delivered by comets that crashed into Earth's surface.    

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