In case of reduction of levels by the height of instrument method,

ANS:D - neither (a) nor (b).

Height of instruments method is;
= Sum Bs - sum fs= last Rl - first Rl. Arithmetic check :
Sum of BS - Sum of FS = Last RL - First RL When we are writing as equation format first-last and last- first matters.
But mentioning as the difference of Last RL and First RL /Last RL and First RL is same. B.S. - F.S. = difference in R.L.S of the first station and last station is correct.