and (B) at least one kind of monomeric units represented by the following formula: ##STR6## wherein R3 stands for a hydrogen atom or a carboxymethyl group, n and m are numbers of zero or 1, with the proviso that when n is zero, m is 1 and R3 is a hydrogen atom and when n is 1, m is zero and R3 is a carboxymethyl group, and the two carboxyl groups may form a carboxylic acid anhydride group.