Once harm ______ to the environment, it takes years to have the system recovered.

A. does　　　　　 B. is done　 C. will be done 　　 D. be done

Once harm ______ to the environment, it takes years to have the system recovered.

A. does　　　 B. is done　 C. will be done 　　 D. be done

Once harm ______ to the environment, it takes years to have the system recovered.

A. does                 B. is done                C. will be done         D. be done

Once harm ______ to the environment, it takes years to have the system recovered.

A. does                         B. is done                        C. will be done   D. be done

Once harm ______ to the environment, it takes years to have the system recovered.

A. does                   B. is done                 C. will be done        D. be done