A type оf flоwering plаnt lives fоr over 50 yeаrs. A lаrge number of the plants are growing in an area. For the first 10 years of their lives, they produce pink flowers. During years 10-20 of their lives, acid rain increases, and the soil pH changes from 7 to 5. In years 20-50 of their lives, the plants produce purple flowers. The change in pH from 7 to 5 resulted in a [y] in hydrogen ions in the soil. The most plausible explanation for the change in flower color of the plants is [x].