Do I say,<br/><br/>(a) The durians fall down when they are ripe.<br/><br/>(b) The durians are ripe when they fall down on / from the tree.<br/><br/>(c) The durians will fall (down) from the tree when they are ripe. <br/><br/>(d) When the duriams are ripe, they drop to / onto / on the ground.