The value of a painting was $1,900 in 1990. Since then, the value of the painting has increased by 15% each decade. Which type of function best models the value of the painting over time? A. An exponential function, because the value is increasing by a constant rate per decade. B. An exponential function, because the value is increasing by a constant percent rate per decade. C. A linear function, because the value is increasing by a constant rate per decade. D. A linear function, because the value is increasing by a constant percent rate per decade.