A storm ________ by a calm.

A. is often being followed

B. was often followed

C. is often followed

D. has often been followed

A. is often being followed

B. was often followed

C. is often followed

D. has often been followed

A storm         by calm.

       A．is often being followed                         B．was often being followed

       C．is often followed                                  D．has often been followed