3.下列制取单质的反应中,化合物作还原剂的是( ) ①用溴和碘化钠反应制取碘 ②用锌和稀硫酸反应制取氢气 ③在电炉中用碳和二氧化硅反应制取硅 ④铝和二氧化锰反应冶炼锰 4.有下列四个含氮物质的反应,其中有一种含氮反应物既是氧化剂又是还原剂的反应是 3. In the following reactions for preparing elemental compounds, compounds are used as reducing agents. (1) Preparation of iodine by reaction of bromine and sodium iodide 2 Preparation of hydrogen by reaction of zinc with dilute sulfuric acid 3 Reaction with carbon and silica in an electric furnace Preparation of silicon 4 aluminum and manganese dioxide reaction smelting manganese 4. There are the following four nitrogen-containing substances in the reaction, including a nitrogen-containing reactant is both an oxidant and a reducing agent