(d) silver
Ans: (d)
25. The main constituents of pearls are: [lAS 1994]
    (a) calcium carbonate and magnesium carbonate
    (b) calcium oxide and ammonium chloride
    (c) aragonite and sodium carbonate
    (d) ammonium sulphate and sodium carbonate
Ans: (a)
26. An element that does not occur in nature but can be produced artificially is :
    (a) thorium
    (b) radium
    (c) plutonium
    (d) uranium
Ans: (c)
27. Which of the following pairs is not correctly matched? [CDS 1993]
         Mineral            Industrial use
    (a)  Zircon             - Nuclear reactors
    (b)  Manganese          - Dry battery
    (c)  Mica               - Bleaching powder
    (d)  Bauxite            - Tanning
Ans: (c)
28. The element found on the surface of the moon is:
    (a) tin
    (b) tungsten
    (c) tantalum
    (d) titanium
Ans: (d)
29. The average salinity of sea water is: [Intelligence Bureau 1994]
    (a) 2%
    (b) 2.5%
    (c) 3%
    (d) 3.5%
Ans: (d)
30. The metallic constituents of hard water are:
    (a) magnesium, calcium and tin
    (b) iron, tin and calcium
    (c) calcium, magnesium and iron
    (d) magnesium, tin and iron
Ans: (c)
31. The high-temperature superconductors are:
    (a) metal alloys
    (b) ceramic oxides
    (c) inorganic polymers
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