Final answer:
In 1970, it took more crude energy to generate electricity than natural gas, as converting natural gas to electricity was only about 40% efficient and coal, used for electricity generation, was more carbon-intensive than natural gas.
Step-by-step explanation:
The relationship between site energy consumption and primary fuel energy consumption in 1970 reflects the efficiency of energy conversion and the type of fuels used. When converting the combustion energy of natural gas into electricity at a rate of 40% through a heat engine, it implies that a significant amount of primary energy in the form of natural gas is used to generate a relatively smaller amount of electricity. Given that coal and natural gas are the two primary fossil fuels for electricity production in the United States, with coal producing nearly twice the carbon emissions of gas combustion, it is energy-intensive to generate electricity, particularly from coal. Therefore, the correct answer to the relationship between site energy and primary fuel energy consumption in 1970 is D: It took more crude energy to generate electricity than natural gas. Based on the information provided, it can be inferred that the answer is A. Electricity was consumed more than natural gas.
In the graph 'Figure 7.4: Recent history of primary energy consumption in the U.S.' it is shown that electricity consumption has been consistently higher than natural gas consumption from 1970 onwards. Additionally, the information provided states that natural gas accounts for around 23% of electricity generation, while coal is primarily used for electricity generation.
Therefore, based on this evidence, option A is the correct answer.