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A)They might eventually cause you to lose sleep. B)They help produce a neurotransmitter in the brain.C)You must not drink milk if you take them.D)They make it unnecessary to take naps.21. A)Characteristics of sand. B)How animals live in the desert. C)A snake’s special way of moving. D)Techniques of skiing.23. A)To climb hills. B)To gain traction. C)To fool its enemies. D)To rest as it moves.24. A)Wavy lines. B)Circular lines. C)Perpendicular lines. D)Parallel lines.25. A)Lower body temperatures. B)Decreased energy consumption. C)Greater ability to conceal itself. D)Wider range of vision