On which interval is the function $f(x)=x^{100}+\sin x-1$ decreasing?

1. $f'(x)=100x^{99}+\cos x$. 2. On $(0,1)$ and $\left(0,\tfrac{\pi}{2}\right)$, both $100x^{99}>0$ and $\cos x>0$, so $f'(x)>0$ (increasing). 3. On $\left(\tfrac{\pi}{2},\pi\right)$, $100x^{99}$ is large positive and dominates $\cos x$, so $f'(x)>0$. 4. The function is increasing on every listed interval, so the answer is None of these. _Source: NCERT Class 12 Mathematics Ch 6 "Application of Derivatives", p.12_