Just playing devil's advocate
why not C?
couldn't we argue that may be that Former US Open champions may be didn't know a great deal about Tennis, because the rackets were so exceptional they could win the championship with out knowing a great deal about the game???
I had selected C as well.
C and D are both not claimed in the passage.
But I guess D is a stronger claim (based on mba's explanation)
and "Former US Open champions know a great deal about Tennis" might be an assumption in the passage?