Indicate whether each of the following descriptions better applies to N-linked (N) or O-linked (O) glycosylation. Your answer would be a four-letter string composed of letters N and O only, e.g. OONO.
( ) It is abundant in proteoglycans.
( ) It involves the attachment of a preassembled block of oligosaccharide onto a protein.
( ) It is attached to a serine or threonine residue in the protein.
( ) It involves heavily sulfated sugars.