If the R.L. of a B.M. is 100.00 m, the back- sight is 1.215 m and the foresight is 1.870 m, the R.L. of the forward station is A. 99.345 m B. 100.345 m C. 100.655 m D. 101.870 m

The correct answer is $\boxed{\text{B. 100.345 m}}$.

The R.L. of a B.M. is the reference level from which all other elevations are measured. The back-sight is the distance from the instrument to the B.M., and the foresight is the distance from the instrument to the forward station. The R.L. of the forward station can be calculated by subtracting the back-sight from the foresight and adding the

R.L. of forward station = R.L. of B.M. – back-sight + foresight

In this case, the R.L. of the B.M. is 100.00 m, the back-sight is 1.215 m, and the foresight is 1.870 m. Therefore, the R.L. of the forward station is:

R.L. of forward station = 100.00 m – 1.215 m + 1.870 m = 100.345 m