Which of the following statements is true of energy sources in the United States? a. Imports of natural gas are cost-effective as natural gas can be transported by ship efficiently. b. Low natural gas prices plus new air-pollution regulations have made coal uncompetitive as a source of electricity. c. The Obama administration approved the construction of the Keystone XL pipeline along the U.S. Gulf Coast. d. The United States is not in favor of using nuclear energy as it releases greenhouse gases. e. U.S. oil production, which rose sharply after 1985, began to decline again in 2009