( ) 15.Tom might have come to school in time for Professor Brown's lecture,______ .

( ) 15.Tom might have come to school in time for Professor Brown's lecture,______ .
A.if he got up early B.unless he had got up early
C.but he got up rather late D.but he had got up so late
if 表示虚拟语气,是否用if he had got up early