Glucose and fructose form

(a). Glucose and fructose form the same osazone, what does it prove? (b). Why more than 3mol "of"PhNHNH_(2) do not react with glucose and fructose and how does PhNHNH_(2), which is a powerful reducting agent, act as an oxidising agent in this case?

Glucose and fructose are

Glucose and fructose are:

Glucose and fructose are :