c$)$ Data from a complete census of $2,000$ farms under wheat crop $($in hectares$)$ in a region are given in the following Table. The farms were stratified on the basis of farm size into seven strata. The total number of farms $(N_i)$ in each stratum is given. The population strata averages for area under wheat per farm $(Y_{Ni})$ and the corresponding standard deviations $(S_i)$ for each stratum are also given:

$\backslash$table$[[$Stratum$,$Farm size $($ha$.),\backslash$table$[[$Total number of$],[$farms $(N_i)$