Let $a,b$ be real numbers between $0\; and\; 1$ such that $z_1=1+bi,z_2=a+i$ and $z_3=0$ forms an equilateral triangle then
$\begin{array}{1 1}(A)a=b=\sqrt 3 -1 \\ (B) a=b=2- \sqrt 3 \\ (C) a= \sqrt 3 -1 ,b= 2- \sqrt 3 \\(D) None\;of\;these \end{array}$