Question

what must be the sum of ∠a and ∠b

Explanation:

Step1: Recall triangle - angle sum property

The sum of the interior angles of a triangle is 180°. In triangle ABC, $\angle A+\angle B+\angle C = 180^{\circ}$.

Step2: Identify the value of $\angle C$

Given that $\angle C = 90^{\circ}$ (right - angle symbol at $\angle C$).

Step3: Solve for $\angle A+\angle B$

Substitute $\angle C = 90^{\circ}$ into $\angle A+\angle B+\angle C = 180^{\circ}$. We get $\angle A+\angle B+90^{\circ}=180^{\circ}$. Then $\angle A+\angle B=180^{\circ}- 90^{\circ}=90^{\circ}$.

Answer:

A. $90^{\circ}$