begin{multicols}{2}
end{multicols}
begin{figure}[ht]
begin{center}$
begin{array}{ll}
includegraphics[width=55mm]{B_-5}&
includegraphics[width=55mm]{B_-10}
end{array}$
end{center}
begin{center}$
begin{array}{rr}
includegraphics[width=55mm]{B_5}&
includegraphics[width=55mm]{B_10}
end{array}$
end{center}
caption{Open $(a) -5%$ $(b) -10% $ $(c) 5%$ $(d) 10%$}
label{Figure. B_Change}
end{figure}
begin{multicols}{2}
end{multicols}