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We learn from the passage that, when sterilisation of water is to be carried out by means of chlorination, ____.
Water of doubtful purity for drinking can be rendered safe
by boiling and then can be cooled in water bags or in
earthenware containers, which must be protected from
dust and flies when boiling is not possible, drinking water
can in many areas be adequately sterilised by
chlorination; one tablet of halazone is added to one litre of
water and allowed to stand for 30 minutes. Water
containing suspended matter should be filtered first.
There is, however the danger of a particularly serious
infectious disease in many regions of Africa, the Middle
and Far East and South America. In these regions the
water of rivers, lakes and canal may be infected, and the
disease is acquired when the water comes in contact with
the skin.