71.Consider the following statements about copper vapour lasers :Through originally invented in 1966, they are now being developed further for a wide variety of applocatopms necaise 1. they can produce short and long wavelength radiations. 2. they can be focussed on very small areas. 3. of their very high pulse repetition frequency and narrow pulse - widths.Of these statements, the correct ones are

72.According to Daltons atomic theory, the smallest particle which can exist independently is

73.Atoms of an element differ from those of all other elements

74.A mixture of iron filings and sand can be separted by

75.The gas that usually causes explosions in coal mines is