50.（）

A.
A．knowing B.B．known C.C．certain D.D．specialized

Walking, if you do it vigorously enough, is the overall best exercise for regular physical activity. It requires no（31）, everyone knows how to do it and it carries the lowest risk of injury. The human body is（32）to walk. You can walk in parks or along a river or in your neighborhood. To get（33）benefit from walking, aim for 45 minutes a day, an average of five days a week.Strength training is another important component of（34）activity. Its purpose is to build and maintain bone and muscle mass, both of which（35）with age. In general, you will want to do strength training two or three days a week, allowing（36）days between sessions.Finally, flexibility and balance training are increasingly important as the body ages. Aches and pains are high（37）the list of complaints in old age. The result of constant muscle tension and stiffness of joints, many of them are（38）, and simple flexibility training can prevent these by making muscles stronger and keeping joints lubricated (润滑). Some of this you do（39）you stretch. If you watch dogs and cats, you’ll get an idea of how natural it is. The general principle is simple: whenever the body has been in one（40）for a while, it is good to briefly stretch it in an opposite position.