what positive development in the last several decades have led to fewer potential brain dead donors than in the past​

Improvements in the safety of transport vehicles and the fight against crime have allowed the number of people with brain death to be less and less over the past few decades.

Explanation:

Traffic accidents and exacerbated violence were two of the biggest promoters of brain deaths worldwide. Although there is still a long way to go to reduce these events even more, the last few decades have shown an efficient decrease in cases of brain death due to improvements in vehicle safety and in the fight against crime, which is the main promoter of violence.

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martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.