GMAT Sentence Correction (SC) | Although the first pulsar, or rapidly spinning collapsed

"To be sighted was observed" is indeed a tricky construction. Remember infinitives can be adverbs, nouns, and adjectives. In this case, the infinitive is used as an adjective to "the first pulsar." The infinitive, like other verbals, carry an idea of action, and the idea of action is that "someone sighted the first pulsar." In passive tense,
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