In a galvanic cell constructed from the half-cells $
\mathrm{Zn}(s) \mid \mathrm{Zn}^{2+}(\text{aq})$
and $
\mathrm{Cu}^{2+}(\text{aq}) \mid \mathrm{Cu}(s)$, which component gains measurable mass as the spontaneous reaction proceeds?
a) the connecting metallic wire
b) the copper electrode
c) the porous plug of the salt bridge
d) the zinc electrode