单项选择题
Passage one
Caffeine not only gives us a daily jump start,but new research suggests it also can strengthen long-term memory.
According to the US Food and Drug Administration,nearly 90 percent of people worldwide consume about 200 milligrams of caffeine each day.That's equivalent to about one strong cup of coffee a day.Writing in"Nature Neuroscience",Johns Hopkins University researchers say their findings show that caffeine strengthens certain memories for up t0 24 hours after being taken in. "We've always known that caffeine has cognitive-enhancing effects(促智效果),but its,particular effects on strengthening memories and making them resistant to forgetting has never been examined in detail in humans,"said senior author of the paper Michael Yassa."We re-port for the first time a specific effect of caffeine on reducing forgetting over 24 hours."Conducting a double-blind trial,the researchers worked with a test group of people who didn't regularly consume caffeinated products.Five minutes after studying a series of images,the test subjects were given either a placebo(无效对照剂)or a 200-milligram caffeine tablet. To check the caffeine levels of their test subjects,the research team took saliva samples(唾液样品)from them before they took their tablets and again one,three and 24 hours after-wards. Both groups of test participants(those who took the placebo and those who took the caffeine tablet)were tested the following day to see if they could recognize the images they'd seen the previous day. The test included showing the test subjects another series of images that included some new images,those that were shown the previous day,as well as other images that were similar,but not the same as those they had viewed earlier. The researchers found that more members of the group who were given the caffeine tablets were able to correctly identify some of the new images as"similar"to previously viewed images rather than incorrectly identifying them as the same. "We also know that caffeine is associated with health and longevity(长寿)and may have some protective effects from cognitive decline like Alzheimer's disease(阿尔茨海默症)."said Yassa. During the research,the researchers did the following things EXCEPT__.
A、giving the test subjects placebos or caffeine tablets
B、checking the caffeine levels of the test subjects
C、encouraging the test subjects to drink more coffee
D、showing the test subjects series of images
