"I would tell anyone who’s thinking about going back to school that it’s not as difficult as you think," says Karen Jonaitis, a woman, last year, at 47, earned a bachelor’s degree in business administration at a college. "I realized that I would not move up in my career without a degree. "A generation ago, Jonaitis would have been a rarity(稀罕)in college. Today about 6 million people aged 25 or older are studying in American institutions of higher learning. Whether they are returning to improve job skills or for the love of learning, adults no longer see age as a deterrent(障碍). Most people jump at the opportunity to do something new. Some of them make changes they’d been thinking about all their lives. Others return to school out of economic necessity.New developments in neuroscience(神经科学)and psychology are confirming that there are few age limits on how much the brain can absorb and for how long--if you stay active. Continuing to learn keeps us mentally in shape and able to learn more. On mental tests, experts discovered, half to two-thirds of the people in their 70s were as intellectually quick as people in their 30s. Those who fell below average tended to be people who had not made a lifelong habit of reading. They also were less physically active.Many adult students say learning is easier for them today. They bring different skills to the classroom, and they’re less upset if something goes wrong. They have a real thirst, a real desire to learn. A life-long habit of reading and physical activities can keep（）in good condition.

A. our mind
B. our body
C. our age
D. our nerves