Explanation:

The angle subtended by the Minor segment of a circle at the center is lesser than the angle subtended by the Major segment at the center of the circle.
The angle subtended at the centre is the angle subtended by an arc or a chord or a sector at the centre of a circle. ... In such cases, the central angle is 180° and the angle subtended at the circle is 90°. So logically, the angle subtended at the semicircle is right angle, that's 90°