m∠BEC + 30° =70°; Substitution Property of Equality.
The correct option is (A).
What is angle?
An angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.
1. Given
2. m ∠ABC = 70°        [Given}
3. m ∠CED = 30°        [Given]
4. m ∠ABC = m ∠BED     [Corresponding angle theorem]
5. m ∠BEC + m ∠CED = m ∠BED    [ addition postulate]
6. m ∠BEC + 30° = 70°     [Substitution property of equality]
7. ∠BEC = 40°             [Subtraction property of equality]
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