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已知集合 $M=\left\{y \mid y=2^x\right\}, N=\left\{y \mid y=\sqrt{1-x^2}\right\}$, 则 $M \cap N=$
A. $\{x \mid 0 < x < 1\}$     B. $\{x \mid 0 < x \leqslant 1\}$     C. $\{x \mid x \leqslant 1\}$     D. $\{x \mid x>0\}$         
不再提醒