The path of one projectile as seen by an observer on another projectile is a/an

Since both projectiles are moving under the influence of gravity. So, relative acceleration of one projectile as seen from other is zero. So, a→=0→ ⇒ axi^+ayj^=0→So, ax=0                    ay=0⇒ dVXdt=0                ⇒ dVYdt=0⇒ VX= constant=K1 (say)⇒ VY= constant=K2 (say)⇒ dxdt=K1                  ⇒ dydt=K2⇒ dx=K1dt              ⇒ dy=K2dt⇒ ∫0xdx=K1∫0tdt        ⇒ ∫0ydy=K2∫0tdt⇒ x=K1t …(1)        ⇒ y=K2t …(2)Divide (2) by (1), we get y=kx  which is the equation of a straight line.