Read the excerpt from A Black Hole is NOT a Hole. Remember that super-pulling space around a black hole—the one with the strongest pull in the universe? That super pull is the close-up gravitational power of a black hole. Here in this zone, the gravity is so intense that nothing can move fast enough to fly, launch, jump, or zoom away. Around this extreme zone is a boundary called the event horizon. This boundary acts like a point of no return. Once past it, there's no going back. Only black holes have such extreme gravity zones, and only black holes have event horizons. This is what makes a black hole different from everything else.

According to context clues, which is the best definition of event horizon?

A. an area with not many events occurring near the horizon
B. an area with many events occurring near the horizon
C. a border surrounding very weak gravity found around a black hole
D. a border surrounding the zone of strong gravity near a black hole