______ is thE CEntEr oF our plAnEtAry systEm wAs A DiFFiCult ConCEpt to grAsp in thE miDDlEAgEs.
A、it is thE sun AnD not thE EArthB、BEing thE sun AnD not thE EArth
C、thE sun AnD not thE EArth
D、thAt thE sun AnD not thE EArth