A、It shows great variance in levels of alertness among volunteers. B、It proves that coffee improves memory and attentiveness. C、

Many say their brain doesn’t start working properly in the morning until they have had a cup of coffee. (30)But the stimulating effect for which caffeine is famous may be an illusion, say researchers. Those who drink a lot of coffee may feel more alert after their first cup of the day. But this is probably because it reverses the fatiguing effects of overnight caffeine withdrawal, say the researchers. Their study suggests coffee drinkers may actually be better off without their habitual morning mug as it raises the risk of anxiety and high blood pressure. In the study, 379 people who abstained from caffeine for 16 hours before drinking either caffeine or a placebo (安慰剂) were tested for a range of responses afterwards. The Bristol University researchers found little variance in levels of alertness among the volunteers. Around half the volunteers drank little or no caffeine and the other half were medium-high caffeine consumers. All were asked to rate their levels of anxiety and alertness and whether they had a headache before and after being given the caffeine or the placebo. They were also asked to carry out a series of computer tasks to test their levels of memory, attentiveness and vigilance. The medium-high caffeine consumers who received the placebo reported a fall in alertness and an increase in headaches, neither of which were reported by those who received caffeine. However, their post-caffeine levels of alertness were no higher than those of no or low-level consumers received a placebo. (32) This suggests caffeine only brings coffee drinkers back up to "normal."
30. What does the passage say about coffee?
31. What does the study suggest coffee drinkers do?
32. What does the study say about caffeine?