1. Cup A contains $m$ milliliters of red ink, and Cup B contains $m$ milliliters of blue ink. Pour $a$ milliliters from Cup A to Cup B $(0<a<m)$, mix well, and then pour $a$ milliliters from Cup B back to Cup A, then at this point
A. The blue ink mixed into Cup A is less than the red ink mixed into Cup B
B. The blue ink mixed into Cup A is more than the red ink mixed into Cup B
C. The blue ink mixed into Cup A is the same as the red ink mixed into Cup B
D. The relationship between the amount of blue ink mixed into Cup A and the amount of red ink mixed into Cup B is uncertain
1. C When $a$ milliliters are poured from cup A to cup B, the concentrations of red and blue ink in cup B become $\frac{a}{m+a}$ and $\frac{m}{m+a}$, respectively. After stirring, when $a$ milliliters are taken from cup B and poured back into cup A, the amount of blue ink in the poured liquid is: $\frac{m a}{m+a}$. At this point, the amount of red ink remaining in cup B is: $a-\frac{a}{m+a} \cdot a=\frac{m a}{m+a}$. Therefore, the two amounts are equal.