SAT Question 145d5ca7: Boundaries answer and explanation

English - Boundaries - No Punctuation Within Clause
0:00
FREE OFFICIAL BANK
Gathering accurate data on water flow in the United States is challenging because of the country’s millions of miles of ______ the volume and speed of water at any given location can vary drastically over time.
13
Difficulty:Medium
Which choice completes the text so that it conforms to the conventions of Standard English?