What is subsidence? 

A)  the failure of the groundwater supply 
B)  the sinking of ground when groundwater has been withdrawn 
C)  the intrusion of saltwater into a freshwater aquifer 
D)  the accumulation of silt behind a dam

Subsidence is the sinking or settling of the ground surface due to the withdrawal of underground water or oil. When groundwater is pumped out, it leaves empty spaces or voids in the soil or rock. As a result, the overlying surface sinks or collapses into the voids. This can cause damage to buildings, roads, and other infrastructure, as well as changes to the landscape and the water cycle.