Choose the correct answers to the questions. 为下列问句选确的答语。

( ) 1. How many minutes do you walk?         

( ) 2. How much exercise do you need?           

( ) 3. What do you do after breakfast?  

( ) 4. How often do you play basketball?    

( ) 5. Do you want to be strong?

A. I need about twenty minutes of exercise.

B. Yes, I do.

C. About five minutes.

D. Three times a week.

E. I usually go to school.

简述荚膜的主要功能。