An enzyme can affect a chemical reaction by bringing the reactant closer together - chemical bonds may be weakened, and reactions will happen faster than without a catalyst. The energy can also be lowered, which means reactions will proceed at a lower temperature, light presence, etc.
The answer is because they are magnetic B.
Solid Magnesium is considered as active metal so it reacts with strong acids like HCl and H₂SO₄ liberating Hydrogen gas according to the following equations:
Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
so the amount of solid magnesium decrease by addition of strong acid to it.