Researcher Lin Zhi developed a process for increasing the tensile strength--measured in gigapascals, or GPa--of silkworm ________ dissolving and reweaving the silk in a solution of iron metal ions, zinc, and sugar, Zhi increased the amount of force required to stretch it from approximately 0.5 GPa to 2 GPa.

Which choice completes the text so that it conforms to the conventions of Standard English?