For a Transverse Wave on a String the String Displacement

Multiple Choice

For a transverse wave on a string the string displacement is described by y(x,t) = f(x-at) where f is a given function and a is a positive constant.Which of the following does NOT necessarily follow from this statement?

A) The shape of the string at time t = 0 is given by f(x) .
B) The shape of the waveform does not change as it moves along the string.
C) The waveform moves in the positive x direction.
D) The speed of the waveform is a.
E) The speed of the waveform is x/t.