The oxidation of D-mannose using Tollens' reagent can convert its aldehyde functional group into a carboxylic acid. Propose the plausible structure of mannuronic acid.

Structure of mannuronic acid showing its aldehyde and carboxylic acid groups.

Structure of a modified mannuronic acid with a hydroxymethyl group.

Structure of a sugar with multiple hydroxyl groups and a hydroxymethyl group.

Structure of a sugar with two carboxylic acid groups.