A wave travelling in positive x-direction with amplitude ( a = 0.2 ) m, velocity ( v = 360 ) m/s, and wavelength ( λ = 60 ) m, then correct expression for the wave is: 
a) (y = 0.2 sin(6π x - 2π t) ) 
b) ( y = 0.2 cos(6π x - 2π t) ) 
c) ( y = 0.2 sin(6π x + 2π t) ) 
d) ( y = 0.2 cos(6π x + 2π t) )