After injecting water into pipe a, the pressure of the closed gas increases, the volume of the gas decreases, the height difference between the liquid levels of a and b, and the height difference between the liquid levels of c and d become larger;
A. The gas volume becomes smaller, and the length of the water column injected into pipe a is greater than the drop distance of the water surface in pipe c, so A is wrong;
B. If the volume of the closed gas remains unchanged, the length of the water column injected into pipe a is equal to d 4 times the rising distance of the water surface of pipe,
Since the pressure of the enclosed gas becomes larger and the volume becomes smaller after the water column is injected into pipe a, the length of the water column injected into pipe a is greater than 4 times the rising distance of the water surface of pipe d, so B is correct ;
The rising distance of the water surface of tubes C and b is greater than the falling distance of the water surface of tube c, so CD is wrong;
Therefore, choose B.