Context: Read the text about passenger pigeons.
Imagine the year 1800. The sky roars with a great clamor, like the sound of a thousand trains. The daytime sky becomes dark as sunlight is blotted out. Is it a terrible thunderstorm? No, it's actually a flock of thousands of passenger pigeons zooming overhead! Hundreds of years ago, there were three to five billion passenger pigeons in America, and the incredible sight of a flock's flight was a regular event. Sadly, passenger pigeons are extinct. None are left today.
Passenger pigeons were a migratory bird. Migratory birds move about from season to season searching for places to nest and feed. The passenger pigeon migration ranged from Canada all the way to Texas and Florida. During a migration, thousands of birds would fly together from morning to night for several days. People reported seeing flocks as large as a mile wide! The migrations took place in spring and fall, when warmer weather brought forests to life with fresh food sources like nuts, seeds, berries, and insects.
At first, it seemed as though passenger pigeons were an endless supply of tasty meat. When a massive flock passed by, hunters could easily catch a few passenger pigeons without affecting the others. But later, hunters would travel to find nesting sites. A passenger pigeon nesting site might contain over one million birds. The birds usually nested close together, and hunters found them to be easy targets.
Eventually this overhunting destroyed the pigeon population. The last known passenger pigeon in the world died at the Cincinnati Zoological Garden in 1914. The pigeon, named Martha, was twenty-nine years old. Although the extinction of the passenger pigeon is a terrible loss, it did have one happy result: it pushed people to create wildlife protection laws to protect other creatures from a similar fate.
Based on the text, which of the following things made the passenger pigeon migration a special event?
A. The migration caused warmer weather and forest growth.
B. Only people in Florida and Texas could see the migration.
C. The migration only happened every one hundred years.
D. The sun was blocked out by huge flocks of birds.
Answer with the option's letter from the given choices directly.

D