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离子交换的运行操作四步骤包括（）、（）、（）和（）。

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强酸、强碱型树脂只有在强酸或强碱条件下才具有较高的交换能力。（）

A. 对
B. 错

Business letter is a formal letter that you send to __________ recipients.

A. internal
B. external
C. both internal and external
D. personal

When you are writing a business letter in a formal style, you may choose the salutation with __________.

A. Dear Mr. John
B. Dear Smith
C. Dear John
Dear Mr. Smith

The closing of an English business letter doesn't consist of __________.

A. complimentary close
B. date
C. signature
D. the typed name