Can someone explain to me what an independent/dependent/control variable is in Psychology? I keep second guessing myself!

A researcher wants to know if memory is better for written words or spoken words. The researcher presents on a computer screen a list of 20 words to the first 30 volunteers/participants to arrive to the lab. The words are presented at a rate of one word per second. After the presentation of the list, each participant of the group is given 5 minutes to write down as many words from the list in any order. The researcher then presents the same list of words at the same rate over headphones to the next 30 volunteers/participants to arrive to the lab. After the auditory presentation of the list, each participant of the group is provided 5 minutes to write down as many words they can recall in any order.