"The cat is black and the dog is white."
Translation:Il gatto è nero e il cane è bianco.
And the weird thing is, that the very next sentence Duo gave me only ed as an option before a word that begins with an i.
It seems that there are several different programmers that enter sentences, and they are in disagreement among themselves.
This is not the first inconsistency I've encountered on Duolingo.
While rare, they are there, and they are annoying...
There is no difference in meaning.
Before a vowel (A, E, I, O, U),
the conjunctions "e" and "o" and the preposition "a" may add a D (ed, od, ad).
In current Italian grammar, this is optional, but is advised if the word following begins with the same letter (ed Elena, od olio, ad Alessandria).
"Od" is used almost exclusively with words beginning with O,
but you will see the others before other vowels ("ed io," for example).