Concept:Use the sine ratio from SOHCAHTOA: sinθ=HypotenuseOpposite.Explanation:In △PQR, the side opposite ∠QPR=30∘ is QR=5cm.The hypotenuse is PQ.So, sin30∘=PQQR=PQ5cm.Given sin30∘=21, we write 21=PQ5cm.Cross-multiply: 1×PQ=2×5cm.Therefore, PQ=10cm.Answer:D. 10.0cm