You buy an A-tranche sequential with only 4 periods (the bond pays monthly) of expected cash flow remaining. The notional amount is $140 million and the total principal received is expected to be (in millions and in chronological order): $25, $32, $38, $45. What is the WAL (weighted average life)? (Do not round any of the inputs.)

a. 2.3 years

b. 0.23 years

c. 0.56 years

d. 0.38 years