Structures in the human body work together to perform specific functions. The
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1) The metal which reduces the other compound is the one higher in the reactivity. So in this case it is .
2) The substance which brings about reduction while itself getting oxidised (that is losing electrons) is called a reducing agent. Here, $\mathrm{Zn}$ is the reducing agent and reduces Cobalt Oxide to Cobalt while itself getting oxidised to Zinc oxide.