一、题目
函数 $f(x)=\left\{\begin{array}{l}\frac{1}{\sqrt{1+x^{2}}}, x \leq 0 \\ (x+1) \cos x, x>0\end{array}\right.$ 的原函数为 ( )
(A) $F(x)=\left\{\begin{array}{l}\ln \left(\sqrt{1+x^{2}}-x\right), x \leq 0 \\ (x+1) \cos x-\sin x, x>0\end{array}\right.$
(B) $F(x)=\left\{\begin{array}{l}\ln \left(\sqrt{1+x^{2}}-x\right)+1, x \leq 0 \\ (x+1) \cos x-\sin x, x>0\end{array}\right.$
(C) $F(x)=\left\{\begin{array}{l}\ln \left(\sqrt{1+x^{2}}-x\right), x \leq 0 \\ (x+1) \sin x+\cos x, x>0\end{array}\right.$
(D) $F(x)=\left\{\begin{array}{l}\ln \left(\sqrt{1+x^{2}}+x\right)+1, x \leq 0 \\ (x+1) \sin x+\cos x, x>0\end{array}\right.$
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