17.The following statement about white is correct ( ). A: It is holy, noble and pure polar color B: Any color system can rely on white as background and foil to achieve coordination effect C: It is the reference coordinate of all hues, chromaticity and lightness D: in the 19th century, superstitious people often offered "three white gifts" to drive away witches, most of which were white food, such as flour, salt, eggs or milk