Which of the following statements are true? (i) a nonhomogeneous system of 7 equations in 6 unknowns always has a unique solu tion (ii) if a and b are row equivalent then their row spaces are the same. (iii) if a 7 × 9 matrix has 7 pivot columns then nul a=r2 a. i only b. ii only c. iii only d. ii and ii only e. i, ii, and iii