Look at the four squares,A ,B ,C , and D, which indicate where the following sentence could be added to the passage．Where would the sentence best fit？
Unless they are flushed or drained from the soil, their buildup may cause a number of serious problems．
One of the most critical soil quality problems related to irrigation is the increase in concentration of dissolved salts．This process, salinization, affects an estimated one-fourth of the world's irrigated cropland．In some places, irrigation water contains as much as 3.5 tons of salt per 1, 000 cubic meters．A As the water flows over and through the ground, it dissolves salts, increasing the salinity of the water．B Since some crops require 6, 000 to 10, 000 cubic meters of water per hectare, land can receive tens of tons of salt per hectare．C As the water evaporates, high concentrations of salts such as sodium chloride are left behind in the topsoil．D Salt buildup can stunt crop growth, decrease yields, kill crop plants, and eventually make the land unproductive．