Answer: smaller pipe
Explanation: for the first alternative that is constructing with bid size pipe which cost total of $115 million throughout the 50 years and a pumping cost which cost $25000 less than the smaller pipe for the next 16 years of which after those years, it will be equal.
While the smaller pipe cost $65million + $100million = $165million then plus the pumping cost which is equally higher than the big pipe cost . Already there is a difference in cost(minus pumping cost)= $165-115= $50million.
And then $25,000 *16 years= $400000 .
So the total difference in cost for the first 16 years is $50.4 million.
So now with interest rate of 8% you'll see that much capital is used in the smaller pipe , so if both pipe system receive interest rate of 8%, the smaller pipe will have more interest than the bigger.