$15\mathrm{.}4($From $[29])$ Suppose that a small pond normally contains $12$ units of dissolved oxygen in a fixed volume of water. Suppose also that at time t $=0,$ a quantity of organic waste is introduced into the pond, reducing the oxygen concentration f$($t$)$ in the pond. After t weeks, the oxygen concentration in the pond is given by$$f$($t$)=\frac{12t^2-15t+12}{t^2+1}$As time goes on$,$ what will be the ultimate concentration of oxygen? Will it retun to $12$ units