Plant Growth and Development

Explanation

2,4-D is a synthetic auxin. Applied broadly it causes uncontrolled, distorted growth in broad-leaved dicot weeds while leaving narrow-leaved monocot crops largely unaffected, which is what makes it a selective herbicide. Kinetin is a cytokinin, and the other two are growth inhibitors or promoters with no herbicidal use.

Explanation

Gibberellin causes bolting — the sudden elongation of the internodes just before flowering — in rosette plants such as cabbage. Abscisic acid is a growth inhibitor, indole butyric acid is an auxin used for rooting, and ethephon releases ethylene, which promotes ripening rather than elongation.

Explanation

Arithmetic growth adds a constant amount per unit time: Lt = L0 + rt. Substituting L0 = 20 cm, r = 30 cm per day and t = 7 days gives 20 + 210 = 230 cm.