Answer the following and briefly explain why

4. Are the following obligations valid, why, and if they are valid, when is the obligation demandable in each case? a) If the debtor promises to pay as soon as he has the means to pay b) if the debtor promises to pay when he likes c) If the debtor promises to pay when he becomes a lawyer. d) If the debtor promises to pay if his son, who is sick with cancer, does not die within one year