Financial reporting rules require liabilities to be reported if they involve "probable future sacrifices of economic benefits," the firm has an unavoidable "present obligation," and the "transaction or event that gave rise to the obligation has already occurred." Considering the different kinds of uncertainty that may exist, what would be the rank order on uncertainty $($most uncertain first$)$ of the following:

Obligations for which the firm must estimate both timing and amount of payment

Contingent obligations

Obligations arising from mutually unexecuted contracts

Obligations with fixed payment dates and amounts

Obligations arising from advances from customers on unexecuted contracts and agreements

Obligations with fixed payment amounts but estimated payment dates

a$.3,6,5,1,4,2$

b$.2,3,6,5,4,2$

c$.1,6,5,3,2,4$

d$.3,5,6,1,2,4$