Context Clues

1.Read the passage then answer the question.

Construction on Florence's grand cathedral began in 1296.Over a hundred years later, however, the structure remained unfinished, with an enormous hole in the roof. The designers had intended to top the cathedral with the world's largest dome, but this plan was stymied by architectural limitation . The main factor stalling progress was the fact that no one knew how to create such a massive copula without large,cumbersome support support systems.

What is the meaning of copula used in the in the passage?

1.dome
2.roof
3.decoration 
4.cathedral