I believe C'est means It Is so, it would be the only answer that can relate.
At first, I thought "une" could be either "a" or "one" and it didn't matter which way you translate it into English. How can I tell if it's supposed to be "one mouse" or "a mouse"? I know "a mouse" may make more sense, but I read it as "That is one mouse!" as if their saying, "It's no big deal. It's just "one" mouse." So how do I tell which way to translate it, because it says "one" is wrong.