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The chemical elements caesium, and rubidium, both of which are metals , were discovered by splitting:
A.Hard ores in a furnace
B.Light in a spectroscope
C.The atom in a cyclotron
D.Salt solution by an electric current

Answer
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Hint:Caesium is a chemical element with the symbol Cs and atomic number 25. It is a soft silvery gold alkali metal with a melting point of 28.5 degree Celsius, which makes it one of only five elemental metals that are liquid at or near room temperature. Rubidium is the chemical element, with the symbol Rb and atomic number 37. It is a very soft, silvery-white metal in the alkali metal group.

Complete step by step answer:
-Caesium was discovered by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff, German scientist, in 1860 through the spectroscopic analysis of Durkheim mineral water. They name cesium after the blue lines they observed in its spectrum.
-Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff, in Heidelberg, -Germany, in the mineral lepidolite through spectroscopy.
-They had developed the spectroscope. The device was based on a prism which separated light from a flame into a rainbow of colors.
-Hence the elements were discovered by splitting light in a spectroscope.
-Obtaining pure caesium is difficult since caesium ores are frequently contaminated with rubidium.
-To obtain pure caesium and rubidium, ores are crushed and heated with sodium metal to 650 degree Celsius, forming an alloy that can then be separated with a process known as fractional distillation.
-For almost 100 years after their discovery, caesium and rubidium were considered laboratory curiosities. Recent reductions in cost have stimulated interest in these metals. Metallic caesium is too reactive to easily handle and is usually sold in the form of cesium azide. Cesium is recovered from cesium azide by heating it.
-Rubidium metal reacts very rapidly in water to form a colorless solution of rubidium hydroxide (RbOH) and hydrogen gas . The reaction is slower than that of caesium (immediately below rubidium in the periodic table) but faster than that of potassium (immediately above rubidium in the periodic table).

Hence, option B is correct.

Additional information:
-Cesium has the second lowest melting point of all metallic elements, which limits its uses.
It readily combines with oxygen and is used as a getter, a material that combines with and removes trace gases from vacuum tubes.
-It is also used in atomic clocks, in photoelectric cells and as a catalyst in the hydrogenation of certain organic compounds.
-Since it is easily ionized and has a high mass, cesium ions may one day be used as a propellant in ion engines on spacecraft.
-It reacts violently with water and ice , forming cesium hydroxide( CsOH).
Rubidium is not particularly harmful to humans, and once in the body its ions are rapidly excreted in sweat and urine.
-Rubidium salts are used in glasses and ceramics and in fireworks to give them a purple colour.
-Rubidium metal reacts very rapidly with water to form a colorless solution of rubidium hydroxide (RbOH) and hydrogen gas. The resulting solution is basic because of the dissolved hydroxide. The reaction is very exothermic.

Note:
Rubidium is a typical but very reactive member of the series of alkali metal .It is appreciably more reactive than potassium, but less than caesium.