We all need (11) ．Doctors say it is good (12) 0ur (13) ．It makes your heart and body strong． And you'll (14) better about yourself． It's best to exercise(15) a week．Twenty minutes each time is enough．

There are (16 ways to exercise． You can walk， (17) ，play football or swim Many people go to special places to exercise． They are called "fitness centers ． These places have (18) equipment． Some people buy equipment(器械) for their families， But it is very expensive．

E．xercising can be fun． Friends can exercise together (19) a fitness center． How do you exercise? How (20) do you exercise?( )11．

A． exercising

B． exercises

C． to exercise

D． exercised