The arrival of anne sullivan was a key event because

It marked the beginning of a new chance for Helen

Explanation:

That was the answer to my test. =)

in ray bradbury's short story "a sound of thunder", mr. eckels's fearfulness leads to drastic consequences for the rest of the world.

in the story, mr. eckels, who is a hunter, hires a time travel company to travel to the age of dinosaurs. although he is excited for the hunting expedition at the beginning, when they reach there and mr. eckels sees the tyrannosaurus, he feels frightened. as he is afraid of the tyrannosaurus, he steps off the path although he has been warned not to do it. by stepping off the path, the entire course of the future is affected: mr. eckart alters the future by stepping on a butterfly, which causes a new man to be elected president in the future. in that way, it is mr. eckart's fearfulness what triggers 'the butterfly effect' and what leads to drastic consequences for the rest of the world.