Why did many native people die in the americas after contact with european explorers?

The europeans had guns and technology

but the thing that killed them the most was the diseases that the europeans brought over that the native people had never experienced before

the native people's immune systems were not use to them so they died
the end

They were exposed to European diseases (i.e. the common cold, measles, etc.) that their bodies hadn't been exposed to before, therefore did not know how to fight, so they were wiped out.  They were also driven out of their land by European settlers, and often died of starvation, hypothermia, etc. when they were forced to move and face new territory they had never seen before.

The answer is option a. boundary line drawn by pope alexander iv in 1493.