$87.$ A thin, flat & square plate measuring $2$ m $2$ m is freely hanging in ambient air at $25$C$.$ It is exposed to the solar radiation falling on one side of the plate at the rate of $500$ W$/$m$2.$ The plate temperature will remain constant at $30$C$,$ if the convective heat transfer co$-$efficient is $\_\_\_\_\_\_\_\_\_\_$ W$/$m$2$ C$.$

$50$

$100$

$150$

$200$