Following are equations of four waves

i)  $y_1=a\sin \omega (t-\frac{x}{v})$          ii) $y_2=a\cos \omega (t+\frac{x}{v})$ 
iii) $z_1=a\sin \omega (t-\frac{x}{v})$        iv)  $z_2=a\cos \omega (t+\frac{x}{v})$
Which of the following statements are correct ?

On superposition of waves (i) and (ii), a travelling wave having amplitude $a\sqrt{2}$ will be formed

On superposition of (iii) and (iv), a transverse stationary wave will be formed

On superposition of (i) and (ii), a stationary wave having amplitude $a\sqrt{2}$ will be formed.

Superposition of waves (ii) and (iii) does not generate a wave.