A recipe calls for 2 cups of flour to make 12 cookies. To find how many cookies can be made with 8 cups of flour, a student sets up a proportion. Which statement explains a common mistake?

Setting up the proportion as $\frac{2}{12} = \frac{c}{8}$ is incorrect because the ratios are not equivalent.

Forgetting to convert cups to a different unit, which is always required.

Setting up the proportion as $\frac{2}{12} = \frac{8}{c}$ is correct, so there is no mistake.

Using division instead of cross-multiplication to solve the problem.

To find the cost of 4 feet of wire if 6 inches cost 30 cents, a student wrote the proportion $\frac{6}{30} = \frac{4}{c}$. Why is this setup incorrect?

The cost unit is not in dollars.

The length units, inches and feet, are inconsistent.

The variable $c$ should be in the numerator.

The proportion is set up correctly.