Sentence:  In September ' 76 Thomas Huxley , Darwin 's famous disciple , came from England to speak in a crowded auditorium at the formal opening of the University ; and although it was a school for men only , it afforded Henrietta an <WSD>opportunity</WSD> to attend its public lectures . 
Meaning of the word marked with <WSD> tags:  a possibility due to a favorable combination of circumstances