What is the purpose of the apron in a proscenium theatre?

a
To conceal the offstage areas on either side of the stage.
b
To create a visual separation between the performers and the audience.
c
To provide additional performance space and allow actors to engage more directly with the audience.
d
To allow for the use of elaborate set designs and scenery.

b

To create a visual separation between the performers and the audience.