Perfect complements are goods that must be consumed in fixed proportions. Examples of perfect complements include __________.

Perfect complements are goods that are consumed together in fixed proportions. Among the examples you provided, the best examples of perfect complements are:

- **Right shoes and left shoes.**

This is because you need one of each to make a complete pair, and they are consumed in a fixed ratio of 1:1.

While "pen and paper" could be considered complementary goods, they are not consumed in fixed proportions like shoes. Similarly, peanut butter and jelly are often used together but can be consumed in varying ratios depending on personal preference. The statement about praise does not fit the context of goods.