Historically, cities have often originated and thrived because they we

Official Explanation:

Step 1:
This is an Assumption question. Determine what unstated assumption is necessary for the argument to make sense.

Step 2:
From 1790 until 1960, the ten most populated cities in the United States all had access to navigable waters. Since 1970, three cities without access to such waters have joined the top ten. All of this is evidence for the author’s conclusion thattrade is not as important a determinant of population as it was in formerdecades.

Step 3:
The evidence is about cities’ proximity to navigable waters; the conclusion is about trade status. The assumption bridges this scope shift: the author assumes that proximity to navigable waters is a necessary condition for a city to become a trade center and, as a result, to grow.

Step 4:
Choice (B) is a good match for this prediction and is the correct answer.
Shifts in navigable waters, choice (A), are irrelevant to the author's argument (even if they actually do occur).
If choice (C), which states that trade doesn’t depend upon actual movement of goods, is true, then the cities that are new arrivals to the top ten may actually be trade centers. But in that case, the conclusion divorcing city growth from trade has nothing to stand on.
Choice (D) may be tempting because it posits that something other than trade has become more important to growth, but it does not correspond to the assumption made in the stimulus—instead, it merely makes the conclusion more specific.
Similarly, choice (E) again posits that something (this time, a shift in population) is more important than trade, but this again merely adds specificity to the conclusion, which already says that trade isn’t as important to city growth as it used to be.

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