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why DoEs thE proFEssor mEntion siriusB、At thE BEginning oF his lECturEA．to introDuCE thE topiC oF his lECturE on whitE DwArFs.
B．to givE A visuAl ExAmplE oF whAt hE is DisCussing.
C．to ExplAin thE rElAtion BEtwEEn stArs AnD thEir CompAnions.
D．to CompArE AnD ContrAst two stAgEs in thE FormAtion oF stArs.