SAT Question 4d13a0c0: Words In Context answer and explanation

English - Words In Context - Fill In Blank Single Word
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Before the Mariner 2 mission completed a successful flyby of Venus in 1962, astronomers’ ideas about the planet were little more than ______. Venus’s atmosphere is so thick that Earth-based observations had yielded very little information about the planet.
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Difficulty:Hard
Which choice completes the text with the most logical and precise word or phrase?