A spherical balloon of radius r meters subtends an angle θ at the eye of an observer. If the angle of elevation of its centre is α, then the height of the centre of the balloon is:

• A. \(r\sin \frac{\alpha }{2}cosec\;\theta\)
• B. \(r\sin \frac{\theta }{2}cosec\;\alpha\)
• C. \(r\sin \theta cosec\frac{\alpha }{2}\)
• D. \(r\sin \alpha cosec\frac{\theta }{2}\)