SAT 1 grammar question: is 'thought as having' grammatically correct ? or should it be 'thought to be' or 'thought of as'? what is the difference

SAT 1 grammar question: is 'thought as having' grammatically correct ? or should it be 'thought to be'  or 'thought of as'?  what is the difference between these three??? Please give examples of each too!!!! (most preferably prev SAT examples) 

1. “thought as having” is grammatically incorrect.2. “thought of as having” is gramatically correct. e.g. : Subjectivity detection or ranking at the document level canbe thought ofas...