Sinkholes and karst topography are a clear indication that ________.
a. the groundwater is moving through cracks in the rock
b. the groundwater is moving through a limestone and forming caves that periodically collapse
c. the area has been subjected to past meteorite bombardments and groundwater is moving through the fractured rock
d. there is nothing unusual; these features occur everywhere
e. there is fast motion of groundwater in the subsurface
