"key answer" is correct and usable in written English.
You can use it when you want to refer to a single answer or solution to a problem that is critical in unlocking greater understanding of the issue at hand. For example: "With the key answer revealed, the mystery behind the strange behavior of the mice was finally solved."
"The key answer is to have a vibrant Apollo".
"If you seek the bathroom key, answer first these riddles three".
And a key answer is that Ebola is shutting down the healthcare system.
Key answer Yes, it seems to me at that time what I was trying to do, having seen Ms Watts' reports on Newsnight I was struck by the similarity to the allegations made in Andrew Gilligan's report and it seemed to me highly likely they had come from the same person.
Such a technique can provide a key answer required for advancing the state-of-the-art in DDoS mitigation and defenses in a realistic environment.
A key answer is the movement of optical technologies to chips themselves by such companies as Luxtera, a venture startup in Carlsbad, Calif., technologies based on Caltech advances that link fiber directly to chips.
If the president is serious about fighting our oil addiction, then developing mass transit is one of the key answers.
Beyoncé's self-portrait is also a declaration of independence, but sung in a more pleasing, feminine key, answering the critics rather than scorning them.
In nation with unrelenting epidemic, an unusual trial may provide key answers.
Our increasing understanding of these epigenetic mechanisms will provide key answers to basic processes in drug addiction and hopefully provide insight into designing improved treatments for drug addiction.
Our study adds two new key answers to this matter: First, we provide evidence that hemofilter passage leads to a reduction in TF levels.
