Answer the following question: Write a multi-choice question for the following article, with the given choices and answer: Article: More than 2.25 billion cups of coffee are drunk throughout the world each day, and it's likely that many taste bitter. Now, a new study suggests that coffee fans can make their drink sweeter by changing the color of their cups. Researchers from Oxford set out to prove whether the claim that coffee drunk from a white cup tastes biter was true. They used 36 volunteers and three different colored cups---blue, white and transparent glass--- to examine the claim. In one experiment, the white cup increased the intensity of the coffee taste relative to the transparent cup. Then, in a second experiment, coffee drunk from the white cup was found to taste less sweet when compared to the other colored cups, while the blue cup made the coffee taste the sweetest. The scientists believe that the color brown may be associated with bitterness, and coffee in a white cup appears the brownest. "Our study clearly shows that the color of a cup does influence our sense of the coffee taste," leading author Dr George Doorn, wrote in an article for The Conversation. "The effect of the color of the cup on the taste of the coffee reported here suggests that cafe owners should carefully consider the color of their cups," he said. "The potential effects may spell the difference between a one-time purchase and a return customer." Actually, the idea that color can change the food and drink taste came out many years ago. A study published last year reported that red, strawberry-flavored cake served on a white plate was rated as 10 percent sweeter than the same food presented on a black plate. Options: A the popularity of sweet coffee. B an experiment made in Oxford. C the effect of cup color on coffee's taste D useful tips on how to make coffee. Answer: C the effect of cup color on coffee's taste Question:
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The passage is mainly about _ .