(1) Sodium hydroxide can react with carbon dioxide to generate sodium carbonate and deteriorate, so it should be sealed and preserved. In order to remove sodium carbonate generated by metamorphism in dilute sodium hydroxide solution, calcium hydroxide solution should be selected, so fill in: CO2+2NaOH═Na2CO3+H2O, sealed, a;

(2) Phenolphthalein solution does not change color, but it may be acidic or neutral. Hydrochloric acid can react with sodium bicarbonate to generate carbon dioxide, and it can react with active metals to generate hydrogen, so it is wrong to fill in: excessive hydrochloric acid makes the solution acidic, but phenolphthalein solution does not change color;

In conclusions B and C of the experimental method (no total score)

ferment