How is the length of the surface calculated in case of countercurrent settling?

• A. Lp=wVƟ/ v cos Ɵ
• B. Lp=(VƟ – V sinƟ)/ v cos Ɵ
• C. Lp=w(VƟ – V sinƟ)
• D. Lp=w(VƟ – V sinƟ)/ v cos Ɵ