Answer :
Final answer:
U-235 is rare because it comprises less than 1% of natural uranium, necessitating a complex and costly enrichment process to increase its concentration for use in nuclear reactors or weapons.
Explanation:
The best evidence to support the statement that U-235 is rare is that U-235 is found in very small quantities in natural uranium. This is correct because naturally occurring uranium is composed of more than 99% uranium-238 and less than 1% uranium-235, which is the fissionable isotope necessary for an atomic bomb.
To use uranium as a fuel for nuclear reactors, its U-235 content must be increased through a process known as enrichment. For nuclear weapons, this enrichment process must result in uranium being enriched to 70% or more of U-235; otherwise, the chain reaction required for an explosion cannot be sustained. The significant effort and resources required to enrich uranium-235 underscore its scarcity.
