Which one of the lists in the options below should replace '???' in the below code fragment to produce the result: [4,8]?

Question

BlackTom AI · Accepted Answer

We’re evaluating which placeholder list, when used as the source of x in the list comprehension [x * 2 | x <- ???, x `mod` 2 == 0], will produce the result [4, 8].
First, consider the idea: x must be even (x `mod` 2 == 0), and then we multiply by 2. Therefore, the resulting list contains twice the even numbers drawn from the source list, in the same order as they appear there.
Option 1: [1, 3, 5]. All elements here are odd, so none satisfy x `mod` 2 == 0. Doubling them would yield [2, 6, 10] if the filter were ignored, but with the condition only even numbers would pass; since there are no even x, the result would be []. This does not match [4, 8].
Option 2: [1, 2, 3, 4]. Among these, the even numbers are 2 and 4. Doubling them gives 4 and 8, respectively, preserving order, so the resulting list is [4, 8], which matches the target.
Option 3: [2, 4, 6, 8]. Every element here is even, so doubling yields [4, 8, 12, 16]. This produces more values than requested and therefore does not match [4, 8].
Option 4: [4, 8]. Both elements are even, so doubling would give [8, 16], not [4, 8], which again does not match the desired result.
In summary, only the second list provides exactly the even numbers 2 and 4 to be doubled, yielding the target [4, 8]. All other lists fail to produce that exact sequence under the given predicate and transformation.

Which one of the lists in the options below should replace '???' in the below code fragment to produce the result: [4,8]? 单项选择题

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