23 Let $z$ and $w$ be complex numbers with modulus 1, and $1 \leqslant|z+w| \leqslant \sqrt{2}$, then the minimum value of $|z-w|$ is
A. 1
B. 2
C. $\sqrt{2}$
D. $\sqrt{3}$
23 C. $|z+w|^{2}+|z-w|^{2}=2\left(|z|^{2}+|w|^{2}\right)=4$, since $|z+w|^{2} \leqslant 2$, hence the minimum value of $|z-w|$ $=\sqrt{2}$.