Which letter-cluster will replace the question mark (?) in the following series?
CED, FJH, KQN, (?) , AKF
I. x2 – 18x + 81 = 0
II. y2 – 3y - 28 = 0
I. 4x² - 15x + 9 = 0
II. 20y² - 23y + 6 = 0
I. x2 – 13x + 36 = 0
II. 3y2 – 29y + 18 = 0
I. 3x2 - 16x - 12 = 0
II. 2y2 + 11y + 9 = 0
I. 5x² + 17x + 6 = 0
II. 2y² + 11y + 12 = 0
...I. 8x² + 2x – 3 = 0
II. 6y² + 11y + 4 = 0
I. x + 1 = 3√ 9261
II. y + 1 = √ 324
I. 117x² + 250x + 117 = 0
II. 54y² -123y + 65 = 0
I. x² + 3x – 154 = 0
II. y² + 5y – 126 = 0
I. 4x2 + 9x - 9 = 0
II. 4y2 - 19y + 12 = 0
