an isthmus is a(n)
a. very tall mountain.
b. unique land formation.
c. body of water.
d. flat grassland region.
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An isthmus is a unique land formation, which is answer choice (b).
To reach this answer, we can eliminate the other options as follows:
a) A very tall mountain does not match the definition of an isthmus. An isthmus refers to a narrow strip of land connecting two larger areas of land, usually with water on both sides.
c) A body of water also doesn't fit the definition of an isthmus. While an isthmus may border bodies of water, it is not a body of water itself.
d) A flat grassland region is not an accurate description of an isthmus. It is more closely associated with plains or prairies, and an isthmus is a specific type of land formation.
Therefore, the correct answer is b) unique land formation.