They were attempting to end the debate over slavery
In a open primary everybody is allowed to vote and in a closed primary on registered people are allowed to vote. Think of it as a Sale at a store. for a closed sale only members are allowed to get the discounts, but in a open sale you get all of the discounts, even if you are not a member.
I'm not 100% certain, but I'm fairly sure that it's because "<span>They realized that people from Africa were more resistant to disease than Europeans were."</span>
Answer:
The age has to be at least 35 years old