Why Would the Gravity Anomaly Over a Deep-Ocean Trench Be

Why would the gravity anomaly over a deep-ocean trench be negative?

A) because the bathymetry of the trench dips very far below sea level
B) because dense crust is being pulled into the trench, leaving less dense sediments above
C) because water fills the space above the subducting slab and is much less dense than rock
D) because the buoyant magma created at the subduction zones is less dense than the rock