What Structures Inside the Human Penis Are Responsible for an Erection

What structures inside the human penis are responsible for an erection?

A) columns of erectile tissue that fill with blood
B) a series of small bones that push out of the pelvic cavity and into the penis during arousal
C) small balloon-like glands that fill with air
D) one large bone that pushes out of the pelvic cavity and into the penis during arousal