Sentence Correction

Among the facts that contributed to cracks in supposedly earthquake-proof defects in buildings in california there were construction defects like steel columns not being welded properly to other factors, welds that they folded too quickly and with too much heat, and a design whose connection configurations were problematic and unreliable.

b.there were construction defects such as steel columns that were not welded properly to other factors, folding welds that they folded

d. were construction defects such as steel columns to which other structural factors had not been welded properly,welds that were folded





answer is d. I choose b why d? plz teach me..[/spoiler]