In $\triangle ABC$, Which of the following is correct?

1. If $\angle ACB$ is an obtuse angle, then $AB^2 > AC^2 + BC^2$
2. If $\angle ACB$ is a right angle, then $AB^2 = AC^2 + BC^2$
3. If $\angle ACB$ is an acute angle, then $AB^2 < AC^2 + BC^2$

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