Why do hikers attempt to climb Mount Everest only during a two-week period each year?

A. There is a two-week period each year when there is an unusual amount of sunshine, making it easier to climb the mountain.
B. The government of Tibet only allows climbers to be in their country for two weeks out of each year.
C. The local villagers consider Mount Everest a god and therefore restrict the time when climbers can go up.
D. There are only two weeks each year when the weather is calm enough for climbing to be safe.