<image> Psychologists found that the students' ability to accept a concept, y, has the following relationship with the time used to present the concept, x (unit: minutes), where 0≤x≤30:
(1) Identify the two variables in the problem;
(2) What is the students' acceptance ability when the time used to present the concept is 10 minutes? 
(3) Based on the data in the table, at how many minutes do you think the students' acceptance ability is the strongest? 
(4) According to the table, within what range of time x does the students' acceptance ability gradually increase? Within what range of time x does the students' acceptance ability gradually decrease?