Is the set {x| x is a difficult class} well defined? (Hint: the answer has nothing to do with the word “specific”; also enough information is given).

1. Analyze the definition of a well-defined set.
2. Determine whether the phrase "difficult class" provides a clear and objective criterion.
3. Conclude that the term "difficult" is subjective and can vary from person to person, thus the set is not well defined.

\{x \mid x \text{ is a difficult class}\} \text{ is not well defined.}