NASA's Mars Pathfinder spacecraft was developed ...

Official Explanation

Answer: A It may be clearer to recognize that a simpler construction such as "less than one-twentieth the cost" would be correct. Here, the words "$3.5 billion real" are just adjectives; they don't change the correctness of the grammar. We can apply the same simplification to each of the other choices to make it more clear that they are wrong. Choice (B): "less than one-twentieth that of the cost." $150 million (what "less than one-twentieth" is modifying) is less than "the cost" not less than "that of the cost." There's nothing for "that of" to stand in for. Choice (C): "less than one-twentieth the costs." We're talking about one price--one cost. Choice (D): The word "than" can only be used once per comparison. "Less than" is not underlined, so introducing another "than" is incorrect. Choice (E): "less than one-twentieth that of the cost resulting..." As with (B), "that of" doesn't have anything to stand in for. It's wrong.

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