Polluted groundwater is difficult to clean up because

a.groundwater is deep in the ground and dispersed through large areas of rock.
b.pollutants cling to the materials that make up the aquifer and contaminate the clean water
c.the recycling process of groundwater can take hundreds or thousands of years
d.All the above

I think its either a. or c. I'm not really sure. Can you please help me out?

my guess would be a or c, too... but d is a possibility

d no doubt

c

Well, it's definitely not b., so it can't be d.
I don't know, I have a feeling it's a., because in c., who would know how long the process would take, and who has been around for hundreds or thousands of years to comfirm that it actually takes that long?