IMO B
just a rephrase of official question.
please refer for explanation
a-recent-report-determined-that-although-only-three-percent-1378.html#p6519
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This is not a rephrase.For a rephrase the solution should have option D rephrased as:
D. Many Guns were reused as a murder weapon in the cases with arrests.
Many guns were resused does not imply that the murderers are the same & hence this is not as supportive an assumption as B.

For the present question both B & D are correct and in fact imply a similar meaning
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I was not sure if it is B or D... I've choosen D.... can someone please explain it??

Let me give it a try:

Even though only 5 percent of people living in Hutorsville own guns, the Hutorsville Police Department reported that 25 percent of those arrested for robberies in Hutorsville had guns. Clearly, a person who has a gun is more likely to commit robbery regularly than a person who does not have a gun.

Lets create a scenario:

In a town of population 100, 5 person own guns. Tom, Dick, Harry, John, and Jeff.

Lets say 4 robbers arrested, one of them was Tom and others were from rest of the population M1, M2,M3.
Above scenario satisfies the 25% fact.

But conclusion says : a person who has a gun is more likely to commit robbery regularly than a person who does not have a gun.

Thus we can discard above mentioned scenario... bcz author assumes involvement of a person who has gun in different robberies.

Another Scenario:
Lets say 100 robberies committed, in 25 robberies Tom, Dick, Harry, John, and Jeff were arrested. 5 robberies committed by each of them.


Lets us analyze option B and D.

(B)People who were arrested for robbery were more likely to commit robberies regularly than people who were not arrested.

We can not tell about non gun holders whether they were also involve in regular crime... may be M1,M2,M3...M75 arrested only once in 75 arrests.
But we can surely say that gun holders regularly committed robbery and were arrested in 25 cases.


(D)Many people who were arrested for robbery have been arrested before.
Now again it is supporting the conclusion that gun holders regularly committed crime.. also option D is not stated in argument under any premise.

"Even though only 5 percent of people living in Hutorsville own guns, the Hutorsville Police Department reported that 25 percent of those arrested for robberies in Hutorsville had guns."

In this question option D and B both are supporting the conclusion.

Further if you refer the similar OG problem on vehicles equipped with radar, you should be able to see the difference in choice B and D.
Choice B is talking about drivers, and choice D is talking about vehicles.
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B is the correct choice..

P1 5 percent of people living in Hutorsville own guns
P2 the Hutorsville Police Department reported that 25 percent of those arrested for robberies in Hutorsville had guns

Conclusion: a person who has a gun is more likely to commit robbery regularly than a person who does not have a gun.

Now if the choice D would have stated that "People who have been arrested have never been arrested before" it would be correct. Applying negation test to this statement " People who have been arrested have been arrested before" would have broken the conclusion. But the choice D is quite different and hence cannot be the correct answer as it would not pass the negation test.

Hope this solves the discrepancy surrounding between choices B and D.
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I chose B because:

If people who were arrested for robbery were less likely to commit robberies regularly than people who were not arrested, it wouldn't matter if they owned a gun or not because they would more than likely not be interested in going out and robbing. This is the opposite of what the H. Police Dept states. Thus, correct.