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单项选择题

The most important task to learn at the concrete operational stage which spans between 7 to 12 years of age is().
A、object permanence and goal-directed behavior
B、the theory of mind and learning self control
C、conservation
D、abstract reasoning and hypothetical thought

问答题

精馏和吸收两个单元操作都属于     过程，区别在于精馏造成两相物系的方法是      ，吸收则是     ，而且精馏利用组分间        的差异实现两相的分离，而吸收利用         的差异。