Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Q: Problem: a train of 25 carriages, each of 60 meters length, when an engine also of 60 meters length is running at a speed of 60 kmph. in what time will the train cross a bridge 1.0 km long ?
Options: a. 2.5, b. 3, c. 5, d. 7, e. 9

A: a
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Q: Problem: 4, 12, 48, 240, 1440, 10080,80640 (... )
Options: a. 7620, b. 10080, c. 6200, d. 10020, e. 725760

A: e
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Q: Problem: if the cost price of 50 articles is equal to the selling price of 20 articles, then the gain or loss percent is ?
Options: a. 16, b. 127, c. 12, d. 18, e. 150

A:
e
****