Susan has the option of purchasing one of three products: Brand A, Brand B, or Brand C. Each costs twenty dollars. If she decides that Brand A meets her needs best, then the opportunity cost of this decision is A) Brand A B) forty dollars. C) Brand A and Brand C D) Brand B or Brand C depending on the value.

Brand B or Brand C depending on the value.

Explanation:Susan gives up the opportunity to buy options B and C in order to buy Brand A. Therefore, depending upon the value of the other two brands, the opportunity cost of Susans decision is not being able to buy Brands B and C.

You mean, biologically? There would be no life, at least not as we know it. Plants need carbon dioxide. I suppose an ecosystem composed entirely of animals might be possible. But animals produce CO2, methane, and a host of other off-gases. So your hypothetical planet would be populated by things very different from us.