If $x:y = 2:1$ and $y:z = 2:1$, which combination is correct?

1. $x, y, z$ are continued proportional
2. $z:x = 1:4$
3. $y^2 + 2x = 4yz$

• ai and ii
• bi and iii
• cii and iii
• di, ii and iii