Sanitary bacteriological research on water by the membrane filter method revealed two red colonies on a membrane filter (Endo agar) through which 500 ml of analyzed water were passed. Calculate the coli index and coli titer of the analyzed water:

The correct answer is A: 4 and 250. Coli index is calculated by dividing the number of red colonies by the volume of water analyzed, which gives 2 red colonies / 500 ml = 4. Coli titer is the reciprocal of the coli index, so it is 1 / 4 = 250. This indicates the concentration of coliform bacteria in the water sample. Choices B, C, and D are incorrect as they do not follow the correct calculation steps for determining the coli index and coli titer. B has the correct coli index but incorrect coli titer. C and D have the values reversed, leading to incorrect results.