hard · Enhanced ACT reading

Identify the punctuation error in the following sentence: 'The structural integrity of the bridge was compromised by two main factors—corrosion of the primary steel cables, and shifting of the foundation bedrock.'

1. The single dash acts as a 'HALF-STOP' and is incorrect because the preceding clause is not grammatically independent.
2. A colon must replace the dash to properly introduce the list of factors, as dashes are only for mid-sentence interruptions.
3. The comma is required to prevent a 'run-on' between the two independent clauses describing the factors.
4. The comma after 'cables' is unnecessary because it incorrectly splits a compound predicate or a simple list of two items.

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