In question c-f. .Why do they calculate $t=3$? When they say $t=4$ at the beginning and then next year is $t=4+1=t=5$
If there are 4 years to maturity now, it will only be 3 years to maturity next year. E.g., initial year is 2000, bond matures in 2004. In 2001, it’s 3 years to 2004.