The “pacemaker” of the heart is called ________________ and is located in the _____________.

A perfectly competitive firm will earn a positive short-run economic profit if ____________.

As blood passes through the pancreas, an increased level of blood glucose will stimulate the release of insulin.  This is an example of :

In the United States, what is most often used to make ethanol?

What are the three challenges to solar energy?

In 2013, renewable energy provided 10% of the total energy used in the United States, and wind energy accounted for 17.4% of the renewable energy used. What percent of the total energy used in the United States in 2011was wind energy?

Solar space heating units tend to be less expensive than solar water heating units because

One of the most common sources of phosphate pollution that has led to eutrophication has resulted from the use of

Which of the following are used by individual farmers to generate biogas for their own use?

According to the Environmental Protection Agency, the greatest current water-pollution challenge in the United States is