Equation of the progressive wave is given by : \[y=a\sin \pi (40t-x)\] where \[a\] and \[x\] are in metre and \[t\] in second. The velocity of the wave is [KCET 1999]

• A.            80 m/s                                    
• B.            10 m/s
• C.            40 m/s                                    
• D.            20 m/s