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After electrifying, bubbles appear on the electrode. The less gas produced is oxygen, which can support combustion and rekindle the wood with Mars. Most of the gas produced is hydrogen, which can be burned.

So water is electrolyzed to produce hydrogen and oxygen, which proves that water is composed of hydrogen and oxygen.

So the answer is as follows: (3) Ignite test tube A, and check test tube B with wooden strips with Mars; Re-ignite the batten with Mars;

(2) Hydrogen and oxygen; Hydrogen and oxygen.