Why Was the City of Constantinople Difficult to Capture in War

Why was the city of Constantinople difficult to capture in war?

A) It was located on a high mountain bluff only accessible by one well-guarded road.
B) It had excellent fortifications and walls that were kept in good repair.
C) It sat on a peninsula that was protected by the sea on three sides and a well-guarded causeway on the fourth side.
D) The region surrounding it was largely barren, making it difficult for attacking armies to feed and support themselves.