The function f is defined by ~$f\left ( x \right )=\sqrt{x}-10$~ for all positive numbers x.  If u = f(t) for some positive numbers t and u.  What is t in terms of u?

~$\sqrt{u+10}$~

~$\left ( \sqrt{u}+10 \right )^{2}$~

~$\sqrt{u^{2}+10}$~

~$\left ( u+10 \right )^{2}$~

~$\left ( u^{2}+10 \right )^{2}$~