Question

the postal railway system delivered mail across the united states for decades, therefore mail trucks replaced mail trains in the late 1970s.
a therefore
b however
c when
d but

Explanation:

To solve this, we analyze the logical relationship between the two clauses. The first clause says the postal railway system delivered mail for decades, and the second says mail trucks replaced mail trains in the late 1970s. This is a contrast (a change or opposing situation), so we need a contrast - indicating conjunction.

• Option A "therefore" shows a cause - effect relationship, which is not appropriate here as there's no causal link between the two events (the railway system being used for decades doesn't cause trucks to replace trains).
• Option C "when" is used to indicate time, but the second clause isn't describing a time - related action relative to the first in the way "when" requires.
• Option D "but" is a conjunction, but in this context, the structure of the sentence (with a comma before the blank) makes "however" (which is an adverb used to show contrast and is often preceded by a semicolon or a period, but in some cases, a comma) a better fit for the contrast between the two ideas. "However" correctly shows the contrast between the long - term use of the postal railway system and the later replacement by mail trucks.

Answer:

B. however