题目内容：
Questions 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___36_____risk of injury．The human body is designed to wall．You can walk in parks or along a river or in your neighborhood．To get___37_____benefit from walking, aim for 45 minutes a day, an average of five days a week．
Strength training is another important___38_____of ．physical activity．Its purpose is to build and____39____ 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, ___40_____recovery days between sessions．
Finally, flexibility and balance training are___41_____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___42_____, and simple flexibility training can____43____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 ___44_____is simple: whenever the body has been in one position for a while, it is good to ___45_____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(36)题应填__________
参考答案：
答案解析：