Read the passage.

But it proved a wet, ungenial summer, and incessant rain often confined us for days to the house. Some volumes of ghost stories, translated from the German into French, fell into our hands. … “We will each write a ghost story,” said Lord Byron; and his proposition was acceded to. There were four of us.
According to the introduction to Frankenstein by Mary Shelley, what motivates the four friends to write ghost stories?

a. sharing volumes of ghost stories when the electricity is down
b. reading volumes of ghost stories while confined to the house
c. sharing experiences of direct encounters with ghosts
d. uncovering volumes of ghost stories during a hail storm

We know that the friends are confined to the house because it says that the "rain confined us for days to the house." It also says that "volumes of ghost stories....fell into our hands". These two details show that the friends have decided to write ghost stories because of their confinement and reading choices. We don't have any information of ghost encounters, a hail storm, or a power outage.