Which of the following is true related to establishing a credit loss allowance for available$-$for$-$sale debt securities$?$

a$.$

Credit losses can exceed total unrealized losses.

b$.$

Credit loss evaluation considers changes in fair value after the reporting date.

c$.$

Credit loss evaluation considers the length of time that fair value has been less than the amortized cost.

d$.$

A credit loss is recognized through an allowance account, permitting reversals of previously recognized credit losses through net income in the period they occur, not through a direct write$-$down of the basis.