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Silver is the best conductor of electricity, copper _____ is closely.
  A. followed      B. to follow      C. following       D. be followed.

Silver is the best conductor of electricity, copper _______ it closely.

A. followed     B. following      C.  to follow    D. follows

Silver is the best conductor of electricity, copper       it closely.

       A．followed   B．to follow  C．following  D．being followed

Silver is the best conductor of electricity, copper _______ it closely．

A． followed            B． following          C． to follow            D． follows

Silver is the best conductor of electricity, copper　　　 it closely.

　 A．followed　　 B．to follow　 C．following　 D．being followed