They change shape due to a number of reasons.
The easiest reason to understand why they change is that they erode.
The coasts around the the world erode due to waves hitting cliffs, the wind batting against the coasts etc. Over a long period of time we can see that they change in shape.
(sorry I don't know why they move)
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The poem "The Negro Speaks of Rivers" is made by Langston Hughes where likened his soul as deep as rivers. The term dusky was associted in the poem to rivers which he described repeatedly throughout as ancient and old. In this poem, the term dusky is referred to the age of the river. Answer is b. age.
