The audience for the new opera, an inventive rendition of Wagner's Tristan and Isolde, was clearly enthralled by the show, for the attendees paused for some moments before applauding vigorously at its conclusion.

A. As underlined
B. were clearly enthralled by the show, for they
C. were clearly enthralled by the show, as they
D. was clearly enthralled by the show, for they
E. was clearly enthralled by the show, as is clear since its attendees

The OA is A, but I am confused, what kind of construction is this 'for the attendees', can somebody throw more light on this kind of construction please?
Experts!! Help!