B. They were killed during a war.
The family was reasonably happy--playing ball outside with the dog. But their happiness may be seen as a kind of complacency since they were unaware of the danger of nuclear war. We must ask ourselves is having all the labor-saving devices, living in a beautiful modern home, and having a happy family life is truly enough to live a good life. Their lives were snatched away from them, possibly because of their ignorance and complacency in the face of the nuclear threat.