Answer:
3. Rivers still provide viable routes for shipping, but it is no longer the most feasible method for moving materials.
Step-by-step explanation:
For a sentence to have a logical transition, there must be an accurate subject-verb or subject-pronoun agreement. It as a pronoun can be used as a subject or object in a sentence. When It is used to describe a subject, the subject is usually in the singular form. For example,
1. It is a book.
The sentence, 'Books are a good source of earning, but it is going extinct' is not correct because there is no agreement between the subject which is in the plural form and the pronoun.
So the third sentence should be worded as 'Rivers still provide viable routes for shipping, but they are no longer the most feasible method for moving materials'.