Figure $1$ shows a phase diagram of a $Cu-Ni$ alloy which having a complete solid solubility in each element. Atomic radius, crystal structure, electronegativity, and valence for $Cu$ and $Ni$ atoms are tabulated in Table $1.$

$($a$)$ Name with justification the category of binary system of the phase diagram shown in Figure $1.$

$($b$)$ Explain TWO $(2)$ ideal conditions for $Cu$ and $Ni$ to produce a complete solubility in each element as shown in Figure $1$ and Table $1.$

$($c$)$ Cite THREE $(3)$ variables that determine the microstructure of a $Cu-Ni$ alloy.

$1$

Table $1$: Data of atomic radius, crystal structure, electronegativity and valence

$\backslash$table$[[$Element$,\backslash$table$[[$Atomic radius$],[(nm)$