离子交换的运行操作四步骤包括()、()、()和()。
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