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A．There is experimental work to show that a week or ten days may not be long enough and a fortnight to three weeks is probably the best theoretical period.
B．Different forms, changing at different rates and showing contrasting combinations of characteristics, were present in different areas.
C． For example, it may be necessary for the spider to leave the branch on which it is standing, climb up the stem, and walk out along another branch.
D．For our present purpose, it seems useful to distinguish two kinds of chemical reaction, according to whether the reaction releases energy or requires it.