You Are an Engineer at a Power Plant That Creates

You are an engineer at a power plant that creates large volumes of hot waste water, which disrupts the ecosystem of the river it drains into. Another part of the plant requires warm oil, and the plant uses large amounts of energy to heat it. Inspired by countercurrent exchange in whales, you save energy and benefit the environment by doing what?

A) running the oil pipes under the river after dumping in the hot waste water
B) running the oil pipes around the waste water pipes, to allow heat transfer from water to oil
C) running the waste water and oil through a single pipe
D) adding a sensor, integrating center and effectors to control temperature in both pipes