Concept:An angle formed in a semicircle is a right angle, so the angle at Q is 90∘.Explanation:Since PR passes through the centre O, it is the diameter of the circle.Therefore, ∠PQR=90∘, by the angle in a semicircle theorem.In △PQR, the sum of interior angles is 180∘.So, x∘+90∘+∠QRP=180∘.Solving gives ∠QRP=180∘−90∘−x∘=(90−x)∘.Answer:∠QRP=(90−x)∘, that is option B.
