Identify the infinite set among descriptions: Which of the following sets is infinite?

Introduction / Context:
An infinite set has no finite bound on its elements. Geometric constructions often yield infinite families (e.g., circles with the same center and varying radii).

Given Data / Assumptions:

• Even prime numbers: only {2} (finite)
• Rivers in one country: finite list
• Concentric circles about a fixed center: radius can vary over infinitely many positive values

Concept / Approach:
Check whether there is a natural limiting count. Concentric circles admit unboundedly many radii.

Step-by-Step Solution:
(a) = {2} → finite(b) Finite by geography(c) Infinite family by any distinct positive radius

Verification / Alternative check:
Between any two radii, more radii exist (continuum), reinforcing infinitude.

Why Other Options Are Wrong:
They are finite sets by definition or context.

Common Pitfalls:
Misinterpreting 'even prime' as many primes; there is only one even prime.

Final Answer:
Set of all concentric circles