Which of the followings are correct?單選或多選:a. Auxins stimulate stem elongation, phototropism and gravitropism.b. Cytokinins promote plant cell division.c. Gibberellins, combined with auxins, can stimulate fruit development without pollination.d. Abscisic acid inhibits growth and keeps seeds in dormancy.e. Ethylene released from ripened fruits can stimulate other fruits to ripen.