A $50wt\%Ni-50wt\%Cu$ alloy is slowly cooled from $1400C(2550F)$ to $1200C(2190F).($Use $Ni-Cu$ Phase Diagram$)$

$($a$)$ At what temperature does the first solid phase form?

$($b$)$ What is the composition of this solid phase?

$($c$)$ At what temperature does the liquid solidify?

$($d$)$ What is the composition of this last remaining liquid phase?