Prestes into the twenty-first century: contemporary

This one is tricky.

Explanation:

While all options do have the tendency to to extend on their own with time, if say the one to extend the longest, as sad as it sounds, is option B. The social boundaries imposed on certain races.

At least that's my best answer. ♥

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).