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Since ∠a and ∠b are supplementary angles, their measures add up to 180 degrees.
m∠a + m∠b = 180°
Since m∠b = 121°, we can substitute this value into the equation:
m∠a + 121° = 180°
To find m∠a, we can isolate it by subtracting 121° from both sides of the equation:
m∠a = 180° - 121°
m∠a = 59°
Therefore, the answer is A. 59°.