单选题 How sleep helps us consolidate memories is still largely a mystery. A recent study from the University of Lǔbeck, in Germany, offers one clue. Subjects were given a list of 46 word pairs to memorize, just before sleep. Then when they reached the deepest stages of sleep, electrical currents were sent through electrodes on their heads to induce very slow brain waves. Such slow waves were induced at random in the brains of one group of subjects, but not another.
The next morning, the slow-wave group had better recall of the words. Other types of memory were not improved, and inducing the slow waves later in the night did not have the same effect. Why and how the slow waves improved memory is not yet understood, but they are thought to alter the strengths of chemical connections, or synapses, between specific pairs of nerve cells in the brain. Memories are "stored" in these synapses: changing the strength of the synapses increases the strength of the memories they store.
Slow waves functioned in the subjects' brains in the experiment when

• A. they were given a list of words before sleep.
• B. they reached the deepest stages of sleep.
• C. they were connected to electrical currents.
• D. they were asked to recall the words the next mornin