2. B, C

3. The water starch added in the test tube turns blue without being digested by iodine. The test tube is a steamed bun block. Starch turns blue when it is not digested by iodine.

4. Prepare test tube cubes. Add the same amount of steamed bread crumbs and 2ml saliva, and fully stir for 0 degree10min. The variable opposite to group A is temperature.