The correct options are
A water
B acid
Dilution of acid is a highly exothermic reaction. Due to this the glassware gets extremely hot and the water spurts out uncontrollably. This can lead to severe burns on skin.
The best practice is to add acid in a dropwise manner to a comparatively large amount of water. So, the heat generated gets dissipated in water to a large extent, avoiding the spurting of acid.