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答案：B
解析：
To find what fraction by mass of the new solution is alcohol, you can find how many grams of alcohol are in the final solution of 20 grams. Since the 8-gram solution is 1 part alcohol to 3 parts water, it is 1/4 alcohol. So it contains 2 grams of alcohol. Similarly, the 12-gram solution is 1/3 alcohol by mass, so it contains 4 grams of alcohol. Thus, the final 20-gram solution contains 6 grams of alcohol, so 6/20, or 3/10, of it is alcohol.
