多项选择题 信仰选择的标准包括（ ）。

不定项选择 In the experiment of purification of sodium chloride, when barium chloride is added into the solution, white precipitate is formed, which indicates the existence of sulfate ion in the solution.

单项选择题 必须按照工艺流程操作，每一位学员必须做好个人防护才允许操作 A、正确 B、错误