A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is

• A. stationary w.r.t. stator
• B. rotating at double the Ns w.r.t. rotor
• C. rotating at -Ns w.r.t. rotor
• D. stationary w.r.t. rotor.