Which of the following geologic materials would have the highest groundwater velocities and be least effective in removing unwanted pollutants from the water?

A. limestone; numerous solution channels and fractures widened by dissolution
B. sandstone; well-cemented with a few, widely spaced vertical fractures
C. unconsolidated medium-grained sand; sand grains are coated with clay minerals
D. lakebeds; unconsolidated very fine-grained sand and silt