(a) Platinum, gold, and silver are used to make jewelry.

(b) Sodium, potassium, and lithium are stored under oil.

Answer:

(a) Because platinum, gold, and silver are highly reactive metals, they are unaffected by air, water, or most chemicals when used to produce the jewelry. These metals have a high sheen and are malleable and ductile in nature, as well as having good corrosion resistance.

(b) Sodium, potassium, and lithium release a lot of heat when they combine with water. As a result, the reaction produces hydrogen, which causes a fire. They react with moisture (water droplets) in the atmosphere when exposed to water, hence these metals are stored under oil to avoid contact with water.