SAT Question 89fbc3eb: Boundaries answer and explanation

English - Boundaries - Semicolon Between Independent Clauses
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The Mission 66 initiative, which was approved by Congress in 1956, represented a major investment in the infrastructure of overburdened national ______ it prioritized physical improvements to the parks’ roads, utilities, employee housing, and visitor facilities while also establishing educational programming for the public.
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Difficulty:Medium
Which choice completes the text so that it conforms to the conventions of Standard English?