The correct answer is A. Keeping it exposed to air.
Slacking of lime is a chemical reaction that occurs when calcium oxide (CaO) is exposed to water. This reaction produces calcium hydroxide (Ca(OH)2), also known as slaked lime. The slaking process is exothermic, meaning that it releases heat.
The rate of slacking is affected by the surface area of the calcium oxide. The smaller the particles, the faster the reaction will occur. This is because there is more surface area
for the water to react with.The presence of air also affects the rate of slacking. Air contains carbon dioxide, which can react with calcium hydroxide to form calcium carbonate (CaCO3). This reaction is endothermic, meaning that it absorbs heat. As a result, the presence of air will slow down the rate of slacking.
Crushing the lime lumps will increase the surface area of the calcium oxide and therefore increase the rate of slacking. However, this effect is relatively small compared to the effect
of the presence of air.Therefore, the correct answer is A. Keeping it exposed to air.