离子交换的选择性
参考答案:
离子交换树脂吸着各种离子的能力不一,有些离子易被吸着,但把它置换下来就困难;另一些离子很难吸着,但置换下来较容易,这种性能称为离子交换的选择性。
When we arrived the cinema, the movie ____________ for 20 minutes.
A. had started B had been on C. started D. is on
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