Please choose the correct answer to the question: In a laboratory activity, four different types of rock were placed in identical acid solutions for one week. Each sample was then measured to see if any of the rock mass was dissolved by the acid solution. Which is the dependent (responding) variable in this activity?

Answer1: the mass of each rock Answer2: the type of rock tested Answer3: the length of time of the test Answer4: the amount of dissolved rock

Answer format: answer1/answer2/answer3/answer4
Answer: answer4