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

• 1 in 100 upward
• 1 in 500 upward
• 1 in 100 downward
• 1 in 503 upward.