采用重结晶提纯样品,要求杂质含量为((____)) 以下,如果杂质含量太高,可先用 (____),( (____))方法提纯。
You can _____ some bottles of wine, or some chocolates, or a bunch of flowers.
— ___________________
— Once or twice a week.
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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