Final answer:
The specific information needed to calculate the efficiency of the Cosumnes Power Plant in 2018 and compare it to a natural gas power plant is not given.
Step-by-step explanation:
The first law efficiency of the Cosumnes Power Plant in 2018 can be calculated using the formula:
Efficiency = (Useful output energy / Input energy) x 100%
However, the specific values of the useful output energy and input energy for the Cosumnes Power Plant in 2018 are not provided in the given information, so it is not possible to calculate the efficiency and compare it to the natural gas power plant in Problem 2-c without further data.