Travis warns that a mistake could alter american as we know it because

In the story "A Sound of Thunder," Ray Bradbury describes a world where time travel has become possible, and there are companies that sell travel packages to the past. One of those sells a hunting trip, in which people go back to the Late Cretaceous period in order to hunt a dinosaur.

The name of the hunting guide is Travis, and before leaving, he warns visitors that even the smallest of mistakes could alter their world as they know it, because a small change of events could detonate a series of changes that are impossible to predict. In the story, one of the characters steps on a butterfly, which causes enormous changes in the present. This phenomenon is often called the "butterfly effect."

resents dr jekyll

explanation:

that is the 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).