It's "ô". Yes, ô and dù are the same. Ô is the way the North people say umbrella, Dù is the way the South say it
cây = a tree, a stick, a baton, a cane - just about anything long, straight, and solid, and made of wood.
It's not necessary that it made of wood ^^ Any thing long, straight and solid are enough ^^
So, let me get this straight. When we use classifier "đàn" for animals, we stick an "a' in the translation without một or get it wrong. With this classifier, we need the một. Is there any rhyme or reason for this? Maybe it is something simple I fail to recognize.