Confounding vs Extraneous

Participants in a between subjects anxiety experiment all start with different levels of anxiety. Baseline anxiety is:

Study on the effect of gifts on self-esteem (SE): the treatment group all have lower baseline SE than the control group.

Participants are randomly assigned to conditions. Participant gender is:

In a study on gender and job satisfaction, all participants in one group are old and the other are all young. Age is:

Individual characteristics in a study where they are randomly assigned to participate in a study about SAT prep efficacy

Variables of interest are light and puzzle solving speed. The light condition has an easy puzzle. Puzzle difficulty is: