[单项选择]

Any brain exercise is better than being a total mental couch potato. But the activities with the most  (1)  are those that require you to work beyond what is easy and comfortable. Playing  (2)  rounds of games and watching the latest documentary marathon on the History Channel may not be enough. Just as your muscles grow stronger with use, mental exercise keeps your mental skills and memory in tone. But what kind of exercise is best for the brain ？ Here are some suggestions.
Be a lifelong learner. Continuing to learn new things can build and  (3)  the connections between brain cells.
(4)  your brain. Think of all mental activities as a continuous whole. Watching a TV documentary would be on the passive, mildly challenging end of the  (5) , while learning how to converse in a new language would be on the active, very challenging end. When it comes to cognitive reserve,  (6) 
A. endless
B. isolation
C. maintain
D. spectrum
E. evidently
F. navigate
G. timid
H. frantic
I. mentally
J. diverse
K. impact
L. deprive
M. alternative
N. excursion