grammaticality - Can someone please explain the validity of "Noun is." as an answer to a question

I am first language English and currently supporting a Japanese teacher in teaching Japanese students English. I've run across an issue twice where my initial reaction was to call it an error, but it is prolific in their teaching material and I'm starting to doubt myself.

In response to "Which is---?" or "What is ---?" questions the textbook recommends the answer "Answer is." This sounds very wrong to me.

Example.
"Which is higher, Mt. Everest or Mt. Fuji?"
The textbook gives the answer "Mt. Everest is."
I think "Mt. Everest." or "Mt. Everest is higher than Mt. Fuji." are both better, or at least more natural, answers.

"What is the month that comes after July?" "August is."
I think it should be "August.", "August comes after July.", "August is the month that comes after July." etc etc.

I'm hoping someone can explain to me why this is wrong. If it's right, I'm looking for someone to tell me why it feels wrong to me. (I'm from the Canadian East-Coast if its a dialect thing. Not a Newfie or anything but I do say aboat.)