Art is an international trader in rare Persian rugs. In June 2012 he received a delivery of an exceptional rug and wrote to his three most frequent clients, Bon, Con and Dan, saying that he was offering to sell the rug for 2 billion VND.

Bon immediately responded that he accepted the offer of the rug but was only willing to pay 1.5 billion VND. Con also replied immediately, stating that he accepted Art's offer but that he would not be able to pay the full amount before September 2012.

Dan replied that he would pay the offered price, but only if Art could supply an additional four rugs of equal quality. Before receiving any of these replies, Art actually sold the rug to Ek, a local client, for 2.1 billion VND.

Required: In the context of the rules governing the creation of contracts under the Vietnamese law, advise:

(a) as to the precise legal nature of Art's original letter;

(b) whether Bon has any right of action against Art;

(c) whether Con has any right of action against Art;

d) whether Dan has any right of action against Art.