Which word in the passage means the same as 'edge'?

Directions: Read the following passage and answer the question that follows.

Humanity is on the verge of discovering alien life, high-ranking NASA scientists say.
"I think we're going to have strong indications of life beyond Earth within a decade, and I think we're going to have definitive evidence within 20 to 30 years," NASA chief scientist Ellen Stofan said Tuesday (April 7) during a panel discussion that focused on the space agency's efforts to search for habitable worlds and alien life.
"We know where to look. We know how to look," Stofan added during the event, which was webcast live. "In most cases we have the technology, and we're on a path to implementing it. And so I think we're definitely on the road."