A student investigated the reactivity of four different metals, A, B, C and D. The student added each metal to aqueous solutions of each of the metal sulfates and observed whether a reaction took place. Give one observation that would show a reaction took place.

When a more reactive metal is added to a solution of a less reactive metal's salt, a displacement reaction occurs. The more reactive metal dissolves, and the less reactive metal is deposited as a solid. This leads to several possible observations:\n- **Colour change:** The solution might change colour (e.g., blue copper sulfate solution becomes colourless as copper is displaced). The metal itself might change colour as a new metal is coated onto it.\n- **Solid forming:** A new solid (the displaced metal) appears, which might look like a powder or a coating on the original metal.\n- **Temperature change:** Displacement reactions are often exothermic, so the solution might get warmer.\n- **Bubbles/fizzing:** If the metal reacts with water or acid present, bubbles of gas might be seen.