题目内容：
Questions 47 to 56 are based on the following passage.Walking, if you do it vigorously enough, is the overall best exercise for regular physical activity. It requires no equipment, everyone knows how to do it and it carries the 47 risk of injury. The human body is designed to walk. You can walk in parks or along a river or in your neighborhood. To get 48 benefit from walking, aim for 45 minutes a day, an average of five days a week.
Strength training is another important 49 of physical activity. Its purpose is to build and 50 bone and muscle mass, both of which shrink with age. In general, you will want to do strength training two or three days a week, 51 recovery days between sessions.
Finally, flexibility and balance training are 52 important as the body ages. Aches and pains are high on the list of complaints in old age. The result of constant muscle tension and stiffness of joints, many of them are 53 ., and simple flexibility training can 54 these by making muscles stronger and keeping joints lubricated (润滑). Some of this you do whenever you stretch. If you watch dogs and cats, you'll get an idea of how natural it is. The general 55 is simple: whenever the body has been in one position for a while, it is good to 56 stretch it in an opposite position.
A．allowing
B．avoidable
C．briefly
D component
E．determined
F．helping
G．increasingly
H．lowest
I．maintain
J．maximum
K．prevent
L．principle
M．provoke
N．seriously
O．topic
第(47)题__________
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