单项选择题

The Clock That Wakes You When You Are Ready
Are you a real grump (脾气坏的人) in the mornings? Do you wake up every day feeling tired, angry and upset, and all too ready to hit the snooze (瞌睡) button? If so, then a new alarm clock could be just for you.
The clock, called SleepSmart, measures your sleep cycle, and waits for you to be in your lightest phase of sleep before waking you up. Its makers say that should ensure you wake up feeling refreshed (恢复精力) every morning.
As you sleep you pass through a sequence of sleep states - light sleep, deep sleep and REM sleep (快速眼动睡眠) - that repeats approximately every 90 minutes. The points in that cycle at which you wake can affect how you feel later, and may even have a greater impact than how long or little you have slept. Being wakened during a light phase means you are more likely to wake up cheerful and full of life and interest.
SleepSmart records the distinct pattern of brain waves produced during each phase of sleep, via a headband equipped with electrodes (电极) and a microprocessor. This measures electric activity of the wearer's brain, and communicates wirelessly with a clock unit near the bed. You program the clock with the latest time at which you want to be wakened, and it then at the proper time wakes. you during the last light sleep phase before that.
The concept was invented by a group of students at Brown University in Rhode Island after a friend complained of waking up tired and performing poorly on a test. 'As sleep-deprived (剥夺) people ourselves, we started thinking of what to do about it,' says Eric Shashoua, a recent college graduate and now chief executive officer of Axon Sleep Research Laboratories, a company created by the students to develop their idea. They have almost finished a prototype and plan to market the product by next year.
SleepSmart is a clock that
A．enables you to go to sleep.
B．enables you to sleep deeply.
C．enables you to get up early.
D．wakes you up during your last light sleep phase.