题目内容：
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 the46 risk of in- jury. The human body is designed to walk. You can walk in parks or along a river or in yourneighborhood. To get 47 benefit from walking, aim for 45 minutes a day, an average of five days a week. Strength training is another important48 of physical activity. Its purpose is to build and 49 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,50 recovery days between sessions.
Finally, flexibility and balance trainingare51 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 are52 , and simple flexibility training can53 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' 11 get an idea of how natural it is. The general54 is simple: whenever the baby has been in one position for a while, it is good to55 stretch it in an opposite position.
根据以上内容，回答题．
第46题答案是______ 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
参考答案：
答案解析：