The last one is true.
Lets say that wave 1 has function of W1 = A*sin(w*t) and second one
W2 = A*(w*t - 180)
both waves have same amplitude A and same frequency (w=2*pi*f)
in t=0 first wave has value of 0 and second one has value of 0. Their sum is equal to 0.
As the time starts passing first wave's value will grow while second one will decrease and go negative (because of the shape of sinus function). Both of them are changing their value at same rate because of same frequency and their sum will always be 0. Check the graph.
Lets say that wave 1 has function of W1 = A*sin(w*t) and second one
W2 = A*(w*t - 180)
both waves have same amplitude A and same frequency (w=2*pi*f)
in t=0 first wave has value of 0 and second one has value of 0. Their sum is equal to 0.
As the time starts passing first wave's value will grow while second one will decrease and go negative (because of the shape of sinus function). Both of them are changing their value at same rate because of same frequency and their sum will always be 0. Check the graph.