(1) Insoluble impurities in crude salt can be removed by filtration. Dissolving to remove insoluble impurities, filtering, and evaporating the filtrate to obtain crude salt. In the filtration operation, glass rods are used to drain water.

(2) When a vertical line is drawn from t 1℃, the solubility at the intersection with the curve is large, so their solubility order at t 1℃ is KCl solution, magnesium sulfate solution and magnesium chloride solution.

(3) According to the constant solute mass before and after dilution with water, the solute mass in 0.9% physiological saline is 9g, so the solution mass is 9g0.9%= 1000g, so the mass of water to be added is1000g-209g = 791g..

By analyzing the above four groups of solutions, it can be seen from Group ③ that 200g of water can dissolve 72g of salt at most, so Group ① ② is an unsaturated solution and Group ③ is a saturated solution; Therefore, A is correct; (3) and (4) are saturated solutions at this temperature, so the solute mass fraction is the same, and B is correct; At 20℃, the maximum mass of sodium chloride dissolved in 200g water is 72g, and C is correct; At 20℃, the solution in ④ is saturated and the remaining 18g salt is not dissolved. Continue to evaporate at constant temperature 100g water, and at most 36g salt will be dissolved in 100g water, then the solid mass obtained by filtration should be 36g+ 18g = 54g, and d is wrong; So choose abc

So the answer is: (1) evaporation? Drainage? (2)KCl solution, magnesium sulfate solution and magnesium chloride solution? (3)79 1? alphabet