Find the exact value of each of the remaining trigonometric functions of $\theta$. Rationalize denominators when applicable. $\sec \theta=-9$, given that $\sin \theta>0$ Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. $\sin \theta=$ (Simplify your answer, including any radicals. Use integers or fractions for any numbers in the expression.) B. The function is undefined.

Step 1 ：Given that $\sec \theta = -9$, we can rewrite this as $\cos \theta = -\frac{1}{9}$.

Step 2 ：Since $\sin^2 \theta + \cos^2 \theta = 1$, we can substitute $\cos \theta$ into this equation to find $\sin \theta$.

Step 3 ：Substituting $\cos \theta = -\frac{1}{9}$ into the equation, we get $\sin^2 \theta + \left(-\frac{1}{9}\right)^2 = 1$.

Step 4 ：Solving this equation, we get $\sin^2 \theta = 1 - \left(-\frac{1}{9}\right)^2 = 1 - \frac{1}{81} = \frac{80}{81}$.

Step 5 ：Taking the square root of both sides, we get $\sin \theta = \pm \sqrt{\frac{80}{81}}$.

Step 6 ：However, we are given that $\sin \theta > 0$, so $\sin \theta = \sqrt{\frac{80}{81}}$.

Step 7 ：Rationalizing the denominator, we get $\sin \theta = \frac{\sqrt{80}}{9}$.

Step 8 ：Finally, simplifying the radical, we get $\sin \theta = \frac{4\sqrt{5}}{9}$.

Step 9 ：So, the exact value of $\sin \theta$ is $\boxed{\frac{4\sqrt{5}}{9}}$.