For which of the following functions $f(n)$ and $g(n),$ it holds $:f(n)=O(g(n)).$ Every $\log$ below is base $2.$
- $f(n)=2^{k \log n}\;, \quad g(n)=n^k$
- $f(n)=2^n\;, \quad g(n)=2^{2 n}$
- $f(n)=\left\{\begin{array}{ll}4^n & \text { if } n<2^{1000} \\
2^{1000}\; n^2 & \text { if } n \geq 2^{1000}\end{array}\;, \quad g(n)=\dfrac{n^2}{2^{1000}}\right.$
- $f(n)=2^{\sqrt{\log n}}\;, \quad g(n)= (\log n)^{100}$