I took my test a couple of years ago and I remember groaning at some of the ways the test writers tried to trick you up. The "prolonged" blast question was a good example. it's not the Coast Guards job to test your knowledge of Oxford English. In this example, 'long' and 'prolonged' have the exact same meaning, if one were to consider practical usage.
It seems to be a common method (mistake in my, and many others' opinions) of test writing, where if the test material is fairly easy, that they feel like they need to include tricks to make the test harder than the material they are testing. It happens all the time where the concepts are not complicated or difficult to grasp. I've seen examples in tests for Red Cross certification
at sea certs, and internet
examples of EMT, medic, police written tests, etc. Most of the concepts are simple if you've studied the material at all. If you grasp it, one should ace that portion of the test. There's nothing wrong with having a good portion of your test takers ace a test that they've studied for. Still, people feel like if too many people ace a test, they've failed somehow in their test writing. What they should be weeding out is people that didn't study or grasp the concepts, rather then folks who appreciate the difference between 'prolonged' and 'long' in some 1920's version of the English language.
In the above example, if you replaced answer 'b' with "1 prolonged and 1 short blast" the question would be even more valid because if one didn't study, they would have an equal chance in choosing the wrong answer.