Dylann Roof, the 21-Year-Old Accused of Murdering Nine People in a South

Dylann Roof, the 21-year-old accused of murdering nine people in a South Carolina church on June 17, 2015, was able to buy a .45-caliber pistol because:

A) his prior crimes were not properly entered into the criminal records databases used for background checks.
B) his age restricted him from being considered for criminal record background checks.
C) while he had been on a "no-buy" list for roughly two years, he was removed after he was no longer deemed a threat by the FBI.
D) drug charges are not considered offenses that prohibit the purchase of firearms under federal law.