The correct answer is: A. Both A and R is true and R is the correct explanation of A.
Slow sand filters are more efficient in removal of bacteria than rapid sand filters because the sand used in slow sand filters is finer than that in rapid sand filters. The finer sand provides a larger surface area for bacteria to attach to, and the slower flow rate of water through the filter allows more time for the bacteria to be removed.
Option B is incorrect because R is not the correct explanation of A. The finer sand in slow sand filters does not directly cause the filters to be more efficient in removing bacteria. Rather, it is the combination of the finer sand and the slower flow rate that makes slow sand filters more efficient.
Option C is incorrect because A is true but R is false. Slow sand filters are more efficient in removing bacteria than rapid sand filters, but the finer sand in slow sand filters is not the only reason for this. The slower flow rate of water through the filter is also important.
Option D is incorrect because A is false but R is true. Slow sand filters are more efficient in removing bacteria than rapid sand filters, but the finer sand in slow sand filters is not the only reason for this. The slower flow rate of water through the filter is also important.