A:若x=y,则x+5=y+5
B:若a=b,则ac=bc
C:若 ac = bc ,则a=b
D:若x=y,则 xa=ya
别人对你说:“Good luck.”你应该说:( )
如图,四边形ABCD∽四边形A1B1C1D1,AB=12,CD=15,A1B1=9,则边C1D1的长是( )