The reduced levels of the string at the consecutive sight rails A and B are 203.575 m, 203.475 m respectively. If the difference of their R.D.s is 10 m, the gradient of the sewer line is A. 1 in 100 upward B. 1 in 500 upward C. 1 in 100 downward D. 1 in 503 upward

The correct answer is A. 1 in 100 upward.

The gradient of a sewer line is the ratio of the change in elevation to the horizontal distance between two points. In this case, the change in elevation is 10 m and the horizontal distance is 100 m, so the gradient is 1 in 100. This means that for every 100 m of horizontal distance, the sewer line rises 1 m in elevation.

Option B is incorrect because the gradient is upward, not downward. Option C is incorrect because the gradient is 1 in 100, not 1 in 500. Option D is incorrect because the gradient is upward, not downward.