设 $b, k$ 为常数, 则函数 $f(x)=\left\{\begin{array}{l}k x+b, x < 1 \\ \sqrt{1+x^2}, x \geq 1\end{array}\right.$, 可导的充分必要条件是
A. $k=0, b=\sqrt{2}$.
B. $k=\frac{\sqrt{2}}{2}, b=\frac{\sqrt{2}}{2}$.
C. $k=\sqrt{2}, b=0$.
D. $k=\frac{2 \sqrt{2}}{3}, b=\frac{\sqrt{2}}{3}$.
D. $k+b=\sqrt{2}$.